diff --git a/README.md b/README.md
index 5557d72..1bea7b4 100644
--- a/README.md
+++ b/README.md
@@ -271,6 +271,37 @@ the order takes you back to the start of it. And the unit you picked to
 support is marked on the board in a different colour from the unit giving
 the order, because otherwise the second half is written blind.
 
+## Ordering a convoy
+
+The rules have had convoys all along -- fifty of the DATC cases are about
+them, including the paradoxes -- but for a while you could not order one.
+The board offered an army the provinces it could walk to, and a crossing is
+by definition not one of those, so the army's half of a convoy could not be
+written. The fleet's half could, which made it worse: the verb was there and
+did nothing anybody could use.
+
+An army on a coast is now also offered the coasts a chain of crewed seas can
+reach, drawn in blue and with long dashes so a crossing does not look like a
+march. A sea counts if there is a fleet standing in it, whoever owns it. A
+fleet that has not been ordered to convoy still counts: it is a thing that
+could be arranged, which is what the negotiation is for, and the adjudicator
+will bounce the crossing if it was not.
+
+That is deliberately not the question the rules ask when they *judge* a
+convoy, which is whether water could ever get there. That one says yes to
+most of Europe. Offering thirty provinces because a chain of fleets is
+conceivable would be worse than offering none.
+
+Clicking a blue coast writes the move `viaConvoy`, which is not a formality:
+the rules allow a unit to be convoyed to a province it could have walked to,
+and the two orders resolve differently.
+
+The two halves are written separately, as they always have been, and the
+order list says so while only one of them exists -- `A Yorkshire → Norway`
+sits there in red until `F North Sea convoys Yorkshire → Norway` joins it,
+and then both go black. Nothing was added to make that happen. The validator
+already knew.
+
 ## Still to build
 
 - bots strong enough to solo against each other, not only to draw
diff --git a/src/App.css b/src/App.css
index 65a9f11..d08dbb7 100644
--- a/src/App.css
+++ b/src/App.css
@@ -102,6 +102,9 @@ header select {
 /* The unit you picked to support or to carry: solid, and not the colour of
    the unit giving the order, so the two halves are never confused. */
 .region.helping { stroke: #1f6fb8; stroke-width: 7; }
+/* Offered, but only a convoy gets there. Long dashes rather than short:
+   the same invitation, made by sea. */
+.region.open.ferry { stroke: #1f6fb8; stroke-dasharray: 22 10; }
 
 .pip {
   fill: #fffaf0;
diff --git a/src/App.tsx b/src/App.tsx
index 6c5b2f8..9fe30b8 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -22,7 +22,13 @@ import { POWERS, PROVINCES, base, type Power } from './game/map'
 import { canStep, validate, type Order, type Unit } from './game/orders'
 import type { Proposal } from './game/press'
 import { endingSounded, mergeCues, type Cue } from './game/sound'
-import { convoyTargets, convoyable, supportTargets, supportable } from './game/targets'
+import {
+  convoyDestinations,
+  convoyTargets,
+  convoyable,
+  supportTargets,
+  supportable,
+} from './game/targets'
 import { adjustmentFor, centreCount, type AdjustOrder, type RetreatOrder } from './game/turn'
 import './App.css'
 
@@ -104,18 +110,39 @@ export default function App() {
   const units = game.board
   const pos = useMemo(() => ({ board: units, own }), [units, own])
 
+  /*
+   * The coasts the selected army could be carried to.
+   *
+   * Kept separate from the rest of the offering because the board draws it
+   * differently -- a crossing is not a march, and a player who cannot tell
+   * them apart will order one meaning the other -- and because it is what
+   * decides whether the move is written `viaConvoy`.
+   */
+  const bySea = useMemo(() => {
+    if (step.kind !== 'move') return new Set<string>()
+    const unit = units.get(step.at)
+    if (!unit) return new Set<string>()
+    const legs = new Set(reachableFrom(unit).map(base))
+    return new Set([...convoyDestinations(units, unit)].filter((p) => !legs.has(p)))
+  }, [step, units])
+
   const offering = useMemo(() => {
     if (step.kind === 'idle') return new Set<string>()
     const unit = units.get(step.at)
     if (!unit) return new Set<string>()
-    if (step.kind === 'move') return new Set(reachableFrom(unit).map(base))
+    if (step.kind === 'move') {
+      // Where its own legs go, and where somebody's fleets could take it.
+      return new Set([...reachableFrom(unit).map(base), ...bySea])
+    }
     if (step.kind === 'support') {
       return step.from === undefined
         ? supportable(units, unit)
         : supportTargets(units, unit, step.from)
     }
-    return step.from === undefined ? convoyable(units, unit) : convoyTargets(step.from)
-  }, [step, units])
+    return step.from === undefined
+      ? convoyable(units, unit)
+      : convoyTargets(units, unit, step.from)
+  }, [step, units, bySea])
 
   const write = useCallback((order: Order) => {
     setOrders((prev) => new Map(prev).set(base(order.at), order))
@@ -157,7 +184,18 @@ export default function App() {
       return
     }
     if (step.kind === 'move') {
-      write({ type: 'move', at: step.at, to: coastOf(units.get(step.at)!, province), power })
+      /*
+       * A destination its legs cannot reach is a crossing, and saying so is
+       * not a formality: the rules let a unit be convoyed to a province it
+       * could have walked to, and the two orders resolve differently.
+       */
+      write({
+        type: 'move',
+        at: step.at,
+        to: coastOf(units.get(step.at)!, province),
+        power,
+        ...(bySea.has(province) ? { viaConvoy: true } : {}),
+      })
       return
     }
     if (step.kind === 'support') {
@@ -319,6 +357,7 @@ export default function App() {
           selected={step.kind === 'idle' ? null : step.at}
           helping={step.kind === 'support' || step.kind === 'convoy' ? step.from : null}
           offering={offering}
+          bySea={bySea}
           onPick={click}
         />
       </div>
@@ -376,6 +415,7 @@ export default function App() {
           orders={orders}
           step={step}
           illegal={illegal}
+          bySea={bySea.size > 0}
           onAsk={(kind) => setStep(step.kind === 'idle' ? step : ({ kind, at: step.at } as Step))}
           onClear={clear}
           onSubmit={submit}
diff --git a/src/components/Board.tsx b/src/components/Board.tsx
index d1d252e..e9eae2f 100644
--- a/src/components/Board.tsx
+++ b/src/components/Board.tsx
@@ -37,6 +37,7 @@ export function Board({
   selected,
   helping,
   offering,
+  bySea,
   onPick,
 }: {
   units: Units
@@ -54,6 +55,12 @@ export function Board({
   helping?: string | null
   /** Provinces the current step will accept a click on. */
   offering?: ReadonlySet<string>
+  /**
+   * Of those, the ones only a convoy can reach. Drawn differently because a
+   * crossing is not a march: it needs fleets that have been asked, and it
+   * fails in ways a land move cannot.
+   */
+  bySea?: ReadonlySet<string>
   onPick?: (province: string) => void
 }) {
   const ids = Object.keys(PROVINCES)
@@ -82,7 +89,9 @@ export function Board({
                 : helping === id
                   ? 'helping'
                   : reachable.has(id)
-                    ? 'open'
+                    ? bySea?.has(id)
+                      ? 'open ferry'
+                      : 'open'
                     : ''
             }`}
             d={SHAPES[id] ?? ''}
diff --git a/src/components/OrderPanel.tsx b/src/components/OrderPanel.tsx
index 25f7096..8660905 100644
--- a/src/components/OrderPanel.tsx
+++ b/src/components/OrderPanel.tsx
@@ -39,6 +39,7 @@ export function OrderPanel({
   orders,
   step,
   illegal,
+  bySea,
   onAsk,
   onClear,
   onSubmit,
@@ -47,6 +48,8 @@ export function OrderPanel({
   orders: ReadonlyMap<string, Order>
   step: Step
   illegal: ReadonlySet<string>
+  /** Whether any of the offered destinations needs a fleet to get there. */
+  bySea?: boolean
   onAsk: (kind: Step['kind']) => void
   onClear: (at: string) => void
   onSubmit: () => void
@@ -75,7 +78,7 @@ export function OrderPanel({
             )}
             <button onClick={() => onClear(selected)}>Hold</button>
           </div>
-          <p className="hint dim">{hint(step)}</p>
+          <p className="hint dim">{hint(step, bySea ?? false)}</p>
         </>
       ) : (
         <p className="hint dim">Click one of your units.</p>
@@ -100,12 +103,14 @@ export function OrderPanel({
   )
 }
 
-function hint(step: Step): string {
+function hint(step: Step, bySea: boolean): string {
   switch (step.kind) {
     case 'idle':
       return ''
     case 'move':
-      return 'Click where it should go.'
+      return bySea
+        ? 'Click where it should go. The coasts in blue need a fleet to carry it — order the convoy too.'
+        : 'Click where it should go.'
     case 'support':
       return step.from === undefined
         ? 'Click the unit to support — or its own province, to hold it there.'
diff --git a/src/game/orders.ts b/src/game/orders.ts
index d3fd2eb..9b74ef8 100644
--- a/src/game/orders.ts
+++ b/src/game/orders.ts
@@ -331,7 +331,7 @@ export function seaRouteExists(
 }
 
 /** The seas a coastal province touches, whichever coast they are on. */
-function coastalSeas(id: string): string[] {
+export function coastalSeas(id: string): string[] {
   const coasts = PROVINCES[id]?.coasts
   const keys = coasts ? coasts.map((c) => `${id}/${c}`) : [id]
   return keys.flatMap((k) => (FLEET[k] ?? []).filter((n) => PROVINCES[base(n)]!.terrain === 'sea'))
diff --git a/src/game/targets.test.ts b/src/game/targets.test.ts
index 4952f92..261a11a 100644
--- a/src/game/targets.test.ts
+++ b/src/game/targets.test.ts
@@ -1,7 +1,13 @@
 import { describe, expect, it } from 'vitest'
 import type { Power } from './map'
 import { boardFrom, type Unit } from './orders'
-import { convoyable, convoyTargets, supportTargets, supportable } from './targets'
+import {
+  convoyDestinations,
+  convoyTargets,
+  convoyable,
+  supportTargets,
+  supportable,
+} from './targets'
 
 const A = (power: Power, at: string): Unit => ({ power, type: 'army', at })
 const F = (power: Power, at: string): Unit => ({ power, type: 'fleet', at })
@@ -53,18 +59,83 @@ describe('who a unit may support', () => {
 })
 
 describe('who a fleet may carry', () => {
-  it('offers armies on coasts, and nobody else', () => {
+  it('offers armies on the coasts it touches, and nobody else', () => {
     const board = boardFrom([F('england', 'nth'), A('england', 'lon'), A('germany', 'mun')])
     const who = convoyable(board, board.get('nth')!)
     expect(who.has('lon')).toBe(true)
     expect(who.has('mun')).toBe(false)
   })
 
+  it('does not offer an army its own chain cannot reach', () => {
+    // The North Sea has no business being asked to carry Ankara. Offering it
+    // wrote an order the adjudicator threw away without saying why.
+    const board = boardFrom([F('england', 'nth'), A('turkey', 'ank')])
+    expect(convoyable(board, board.get('nth')!).has('ank')).toBe(false)
+  })
+
+  it('reaches further when there are fleets to reach with', () => {
+    const alone = boardFrom([F('england', 'nth'), A('france', 'spa')])
+    expect(convoyable(alone, alone.get('nth')!).has('spa')).toBe(false)
+
+    const chain = boardFrom([
+      F('england', 'nth'),
+      F('england', 'eng'),
+      F('england', 'mao'),
+      A('france', 'spa'),
+    ])
+    expect(convoyable(chain, chain.get('nth')!).has('spa')).toBe(true)
+  })
+
   it('lands them on a coast that is not the one they left', () => {
-    const where = convoyTargets('lon')
+    const board = boardFrom([F('england', 'nth'), A('england', 'lon')])
+    const where = convoyTargets(board, board.get('nth')!, 'lon')
     expect(where.has('nwy')).toBe(true)
     expect(where.has('lon')).toBe(false)
     expect(where.has('mun')).toBe(false)
     expect(where.has('nth')).toBe(false)
   })
 })
+
+describe('where an army may be carried', () => {
+  it('offers a coast across one sea with a fleet in it', () => {
+    const board = boardFrom([A('england', 'lon'), F('england', 'nth')])
+    const where = convoyDestinations(board, board.get('lon')!)
+    expect(where.has('nwy')).toBe(true)
+    expect(where.has('bel')).toBe(true)
+  })
+
+  it('offers nothing when there is no fleet to carry it', () => {
+    const board = boardFrom([A('england', 'lon')])
+    expect(convoyDestinations(board, board.get('lon')!).size).toBe(0)
+  })
+
+  it('counts a fleet of any power, because that is what talking is for', () => {
+    const board = boardFrom([A('england', 'lon'), F('germany', 'nth')])
+    expect(convoyDestinations(board, board.get('lon')!).has('nwy')).toBe(true)
+  })
+
+  it('walks a chain, and stops where the chain does', () => {
+    // London to Spain wants the Channel and the Mid-Atlantic. With only the
+    // Channel crewed it reaches Brest and no further.
+    const one = boardFrom([A('england', 'lon'), F('england', 'eng')])
+    expect(convoyDestinations(one, one.get('lon')!).has('bre')).toBe(true)
+    expect(convoyDestinations(one, one.get('lon')!).has('spa')).toBe(false)
+
+    const two = boardFrom([A('england', 'lon'), F('england', 'eng'), F('england', 'mao')])
+    expect(convoyDestinations(two, two.get('lon')!).has('spa')).toBe(true)
+  })
+
+  it('never offers the army its own province, or anywhere inland', () => {
+    const board = boardFrom([A('england', 'lon'), F('england', 'nth')])
+    const where = convoyDestinations(board, board.get('lon')!)
+    expect(where.has('lon')).toBe(false)
+    expect(where.has('mun')).toBe(false)
+    expect(where.has('nth')).toBe(false)
+  })
+
+  it('has nothing to say about a fleet, or an army inland', () => {
+    const board = boardFrom([F('england', 'lon'), A('germany', 'mun'), F('england', 'nth')])
+    expect(convoyDestinations(board, board.get('lon')!).size).toBe(0)
+    expect(convoyDestinations(board, board.get('mun')!).size).toBe(0)
+  })
+})
diff --git a/src/game/targets.ts b/src/game/targets.ts
index 786a8a6..10299cd 100644
--- a/src/game/targets.ts
+++ b/src/game/targets.ts
@@ -1,6 +1,6 @@
 import { reachableFrom } from './layout'
-import { PROVINCES, base } from './map'
-import type { Board, Unit } from './orders'
+import { FLEET, PROVINCES, base } from './map'
+import { coastalSeas, type Board, type Unit } from './orders'
 
 /**
  * What each half of an order may be clicked on.
@@ -51,24 +51,80 @@ export function supportTargets(board: Board, unit: Unit, from: string): Set<stri
   return out
 }
 
-/** The armies a fleet at sea may carry: any army on a coast but its own. */
-export function convoyable(board: Board, unit: Unit): Set<string> {
+/**
+ * The coasts a chain of crewed seas touches, starting from these.
+ *
+ * The model the whole convoy offering is built on: a sea counts if there is
+ * a fleet standing in it, whoever owns it, and the chain runs as far as the
+ * fleets do. A fleet that has not been ordered to convoy still counts -- it
+ * is a thing that could be arranged, which is what the negotiation is for,
+ * and the adjudicator will bounce the crossing if it is not.
+ *
+ * This is deliberately not the question the rules ask when they *judge* a
+ * convoy order, which is whether water could ever get there. That one says
+ * yes to most of Europe: it would offer thirty provinces because a chain of
+ * fleets is conceivable, which is worse than offering none.
+ */
+function coastsReached(board: Board, from: readonly string[]): Set<string> {
+  const crewed = (id: string) => board.get(base(id))?.type === 'fleet'
   const out = new Set<string>()
-  for (const [at, other] of board) {
-    if (other.type !== 'army') continue
-    if (PROVINCES[at]!.terrain !== 'coast') continue
-    if (at === base(unit.at)) continue
-    out.add(at)
+  const seen = new Set<string>()
+  const queue = [...from]
+
+  while (queue.length > 0) {
+    const sea = queue.shift()!
+    if (seen.has(sea)) continue
+    seen.add(sea)
+    for (const next of FLEET[sea] ?? []) {
+      const p = base(next)
+      if (PROVINCES[p]!.terrain === 'sea') {
+        if (crewed(p)) queue.push(p)
+      } else if (PROVINCES[p]!.terrain === 'coast') {
+        out.add(p)
+      }
+    }
   }
   return out
 }
 
-/** Where it may be put ashore: any other coast. Whether a chain of fleets
- * actually exists is the adjudicator's business, not the map's. */
-export function convoyTargets(from: string): Set<string> {
-  return new Set(
-    Object.keys(PROVINCES).filter(
-      (p) => PROVINCES[p]!.terrain === 'coast' && p !== base(from),
-    ),
-  )
+/** The seas off this coast that have a fleet in them. */
+const putToSea = (board: Board, coast: string): string[] =>
+  coastalSeas(coast)
+    .map(base)
+    .filter((sea) => board.get(sea)?.type === 'fleet')
+
+/** Where an army could be carried to, given the fleets actually on the board. */
+export function convoyDestinations(board: Board, unit: Unit): Set<string> {
+  const here = base(unit.at)
+  if (unit.type !== 'army' || PROVINCES[here]?.terrain !== 'coast') return new Set()
+  const out = coastsReached(board, putToSea(board, here))
+  out.delete(here)
+  return out
+}
+
+/**
+ * The armies this fleet could carry.
+ *
+ * Only the ones its own chain can actually reach. Offering every army on
+ * every coast wrote orders the adjudicator threw away: the North Sea has no
+ * business being asked to carry an army out of Ankara.
+ */
+export function convoyable(board: Board, unit: Unit): Set<string> {
+  const sea = base(unit.at)
+  if (unit.type !== 'fleet' || PROVINCES[sea]?.terrain !== 'sea') return new Set()
+  const ashore = coastsReached(board, [sea])
+  const out = new Set<string>()
+  for (const coast of ashore) {
+    if (board.get(coast)?.type === 'army') out.add(coast)
+  }
+  return out
+}
+
+/** Where this fleet could put that army ashore: the same chain, less home. */
+export function convoyTargets(board: Board, unit: Unit, from: string): Set<string> {
+  const sea = base(unit.at)
+  if (unit.type !== 'fleet' || PROVINCES[sea]?.terrain !== 'sea') return new Set()
+  const out = coastsReached(board, [sea])
+  out.delete(base(from))
+  return out
 }