diff --git a/frontend/src/capture/session.ts b/frontend/src/capture/session.ts
index cb26783..9c83e96 100644
--- a/frontend/src/capture/session.ts
+++ b/frontend/src/capture/session.ts
@@ -10,7 +10,7 @@ import { measure } from '../measure/measure';
 import { orientedRect } from '../geometry/rect';
 import { encodeInventoryPng, type InventoryInput } from '../png/inventory';
 import { encodeOverlayPng, type EncodeOverlayInput } from '../png/overlay';
-import { rotateCalibration } from '../png/rotateCal';
+import { bakeTiltTransform, rotateCalibration } from '../png/rotateCal';
 import { decodeOrientedBitmap } from '../decodeOrientedBitmap';
 import { clampTiltDeg, defaultTape, tapeEndpoints, type TapeTransform } from './tapeGeometry';
 import { getCropFill } from '../settings/cropFill';
@@ -636,19 +636,37 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       // #63: second copy of the calibration inventory PNG with dimension overlays.
       // Keep clean inventoryPng untouched; never stash overlay into batchApply[cal].
       // Overlay uses the same baked orientation as the clean cal PNG (#99 scope).
+      // Length / width directions follow the tape (not wood MAR): map tick A→B
+      // through the same bake as rotateCal, then orient the overlay bbox to that.
       let overlayBlob: Blob;
       try {
+        const preEp = axis.tape.spanPx > 0
+          ? tapeEndpoints(axis.tape, source.widthPx, source.heightPx)
+          : null;
+        const bakeXform = bakeTiltTransform(source.widthPx, source.heightPx, axis.tape.tiltDeg);
+        let overlayBbox = calMask.bbox;
+        let overlayAxis: { start: [number, number]; end: [number, number] } | undefined;
+        if (preEp && preEp.spanPx > 0) {
+          const start = bakeXform.mapPoint(preEp.start[0], preEp.start[1]);
+          const end = bakeXform.mapPoint(preEp.end[0], preEp.end[1]);
+          const dx = end[0] - start[0];
+          const dy = end[1] - start[1];
+          const angleDeg = ((Math.atan2(dy, dx) * 180) / Math.PI + 180) % 180;
+          overlayBbox = orientedRect(calMask.bbox, angleDeg);
+          overlayAxis = { start, end };
+        }
         overlayBlob = await encodeOverlay({
           inventoryPng: pngBlob,
           bitset: calMask.bitset,
           widthPx: calSource.widthPx,
           heightPx: calSource.heightPx,
-          bbox: calMask.bbox,
+          bbox: overlayBbox,
           scale: base.scale,
           lengthIn: eighthsToIn(lengthEighths),
           lengthLabel: fmtEighths(lengthEighths),
           slabBounds: calMask.slabBounds,
           source: calSource,
+          axis: overlayAxis,
         });
       } catch {
         // Deterministic tests may inject a non-decodable inventory blob; still
diff --git a/frontend/src/png/overlay.test.ts b/frontend/src/png/overlay.test.ts
index d3940d9..74519c4 100644
--- a/frontend/src/png/overlay.test.ts
+++ b/frontend/src/png/overlay.test.ts
@@ -1117,3 +1117,110 @@ test('buildOverlayPlan: tapered tips (~20% max width) included; length span ≥
   const tipSpanSrc = tipHi - tipLo;
   assert.ok(tipSpanSrc <= woodSpan + 2, `tips must not overshoot wood (${tipSpanSrc} vs ${woodSpan})`);
 });
+
+test('buildOverlayPlan: when MAR angle ≠ tape angle, length || tape and width ⊥ tape (~2°)', () => {
+  // Wood MAR is horizontal (angleDeg 0); tape / approved axis is vertical (90°).
+  // Without optional axis (or orientedRect to tape), widths would be vertical.
+  // With tape-driven axis, length is vertical and width left→right ≈ horizontal.
+  const W = 240, H = 400;
+  const bitset = new Uint8Array(W * H);
+  // Slightly elongated horizontally so MAR long side ≈ 0°, but still a tall slab.
+  for (let y = 40; y <= 360; y++) {
+    for (let x = 40; x <= 200; x++) bitset[y * W + x] = 1;
+  }
+  const mar = minAreaRect(bitset, W, H)!;
+  // Force wood-long-axis bbox to 0° — intentionally NOT the tape (vertical).
+  const woodBbox = orientedRect(mar, 0);
+  assert.equal(woodBbox.angleDeg, 0);
+
+  // Tape vertical: tick A near bottom, B near top (image y↓).
+  const tapeStart: [number, number] = [120, 350];
+  const tapeEnd: [number, number] = [120, 50];
+  const tapeAngleDeg = ((Math.atan2(tapeEnd[1] - tapeStart[1], tapeEnd[0] - tapeStart[0]) * 180) / Math.PI + 180) % 180;
+  // Overlay bbox keeps MAR extents but direction from tape (session.calculate path).
+  const tapeBbox = orientedRect(woodBbox, tapeAngleDeg);
+  const scale = 10;
+  const plan = buildOverlayPlan({
+    bitset,
+    widthPx: W,
+    heightPx: H,
+    bbox: tapeBbox,
+    scale,
+    lengthIn: 32,
+    lengthLabel: '32"',
+    mapToInventory: (x, y) => [x, y],
+    axis: { start: tapeStart, end: tapeEnd },
+  });
+
+  // Length a→b must be parallel to tape (vertical → |dx| small vs |dy|).
+  const ldx = plan.length.b[0] - plan.length.a[0];
+  const ldy = plan.length.b[1] - plan.length.a[1];
+  const llen = Math.hypot(ldx, ldy) || 1;
+  const lux = ldx / llen, luy = ldy / llen;
+  const tdx = tapeEnd[0] - tapeStart[0], tdy = tapeEnd[1] - tapeStart[1];
+  const tlen = Math.hypot(tdx, tdy) || 1;
+  const tux = tdx / tlen, tuy = tdy / tlen;
+  const lengthDot = Math.abs(lux * tux + luy * tuy);
+  assert.ok(lengthDot > 0.999, `length must || tape (dot=${lengthDot})`);
+
+  assert.ok(plan.widths.length > 0, 'expected width stations');
+  // Tape-perpendicular = horizontal for a vertical tape.
+  for (const w of plan.widths) {
+    const ang = Math.atan2(w.right[1] - w.left[1], w.right[0] - w.left[0]);
+    let deg = (ang * 180) / Math.PI;
+    if (deg > 90) deg -= 180;
+    if (deg < -90) deg += 180;
+    assert.ok(
+      Math.abs(deg) <= 2,
+      `width station ${w.stationIn} left→right must be within 2° of tape-perpendicular/horizontal (deg=${deg.toFixed(2)})`,
+    );
+  }
+
+  // Contrast: same wood bbox WITHOUT tape axis would follow MAR (horizontal length).
+  const woodPlan = buildOverlayPlan({
+    bitset,
+    widthPx: W,
+    heightPx: H,
+    bbox: woodBbox,
+    scale,
+    lengthIn: 32,
+    lengthLabel: '32"',
+    mapToInventory: (x, y) => [x, y],
+  });
+  const wdx = woodPlan.length.b[0] - woodPlan.length.a[0];
+  const wdy = woodPlan.length.b[1] - woodPlan.length.a[1];
+  const wlen = Math.hypot(wdx, wdy) || 1;
+  // Wood MAR length should NOT be parallel to vertical tape.
+  const woodDot = Math.abs((wdx / wlen) * tux + (wdy / wlen) * tuy);
+  assert.ok(woodDot < 0.5, `wood-MAR overlay must disagree with tape (dot=${woodDot})`);
+});
+
+test('buildOverlayPlan: optional axis alone drives direction even if bbox angle wrong', () => {
+  const W = 200, H = 400;
+  const bitset = new Uint8Array(W * H);
+  for (let y = 50; y <= 350; y++) {
+    for (let x = 70; x <= 130; x++) bitset[y * W + x] = 1;
+  }
+  const mar = minAreaRect(bitset, W, H)!;
+  // Wrong bbox angle on purpose (horizontal) while axis is vertical.
+  const wrongBbox = orientedRect(mar, 0);
+  const plan = buildOverlayPlan({
+    bitset,
+    widthPx: W,
+    heightPx: H,
+    bbox: wrongBbox,
+    scale: 10,
+    lengthIn: 30,
+    lengthLabel: '30"',
+    mapToInventory: (x, y) => [x, y],
+    axis: { start: [100, 340], end: [100, 60] },
+  });
+  assert.ok(plan.widths.length > 0);
+  for (const w of plan.widths) {
+    const ang = Math.atan2(w.right[1] - w.left[1], w.right[0] - w.left[0]);
+    let deg = (ang * 180) / Math.PI;
+    if (deg > 90) deg -= 180;
+    if (deg < -90) deg += 180;
+    assert.ok(Math.abs(deg) <= 2, `axis override: width deg=${deg.toFixed(2)}`);
+  }
+});
diff --git a/frontend/src/png/overlay.ts b/frontend/src/png/overlay.ts
index a328861..b1cf9ad 100644
--- a/frontend/src/png/overlay.ts
+++ b/frontend/src/png/overlay.ts
@@ -81,6 +81,12 @@ export interface OverlayPlanInput {
   /** Inventory PNG size — used to clamp labels inside the canvas (#71). */
   inventoryW?: number;
   inventoryH?: number;
+  /**
+   * Optional explicit length axis (e.g. baked tape tick A→B). When set, (ux,uy)
+   * come from this segment instead of bbox corners — keeps width ⊥ tape even
+   * when MAR wood long side differs from the tape.
+   */
+  axis?: { start: [number, number]; end: [number, number] };
 }
 
 /**
@@ -154,6 +160,7 @@ function maskAxisSegment(
   w: number,
   h: number,
   bbox: BBox,
+  axis?: { start: [number, number]; end: [number, number] },
 ): {
   start: [number, number];
   end: [number, number];
@@ -164,7 +171,7 @@ function maskAxisSegment(
   uy: number;
   span: number;
 } {
-  const { start: a, end: b } = axisEndpoints(bbox);
+  const { start: a, end: b } = axis ?? axisEndpoints(bbox);
   const ex = b[0] - a[0];
   const ey = b[1] - a[1];
   const elen = Math.hypot(ex, ey) || 1;
@@ -270,9 +277,9 @@ function fmtInches(inches: number): string {
 export function buildOverlayPlan(input: OverlayPlanInput): OverlayPlan {
   const {
     bitset, widthPx, heightPx, bbox, scale, lengthIn, lengthLabel, mapToInventory,
-    inventoryW, inventoryH,
+    inventoryW, inventoryH, axis,
   } = input;
-  const seg = maskAxisSegment(bitset, widthPx, heightPx, bbox);
+  const seg = maskAxisSegment(bitset, widthPx, heightPx, bbox, axis);
   // Re-orient span ends so EXIF bottom establishes station origin; axis still
   // lies on the approved M1 direction (seg ux/uy flipped only when needed).
   const oriented = pickBottomTowardTop(seg.start, seg.end);
@@ -522,6 +529,8 @@ export interface EncodeOverlayInput {
   source: { widthPx: number; heightPx: number };
   /** Optional override; default reads live settings.crop_fill (#99). */
   cropFill?: number;
+  /** Optional explicit length axis in source px (baked tape A→B). */
+  axis?: { start: [number, number]; end: [number, number] };
 }
 
 /**
@@ -549,6 +558,7 @@ export async function encodeOverlayPng(input: EncodeOverlayInput): Promise<Blob>
       mapToInventory,
       inventoryW: outW,
       inventoryH: outH,
+      axis: input.axis,
     });
     drawOverlayPlan(ctx, plan);
     return await canvas.convertToBlob({ type: 'image/png' });
diff --git a/frontend/src/png/rotateCal.test.ts b/frontend/src/png/rotateCal.test.ts
new file mode 100644
index 0000000..5d03bb5
--- /dev/null
+++ b/frontend/src/png/rotateCal.test.ts
@@ -0,0 +1,56 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { bakeTiltTransform } from './rotateCal.ts';
+
+test('bakeTiltTransform: identity ≈0° leaves points and size unchanged', () => {
+  const x = bakeTiltTransform(800, 600, 0);
+  assert.equal(x.identity, true);
+  assert.equal(x.outW, 800);
+  assert.equal(x.outH, 600);
+  assert.deepEqual(x.mapPoint(100, 200), [100, 200]);
+  assert.deepEqual(x.mapPoint(0, 0), [0, 0]);
+});
+
+test('bakeTiltTransform: 360° folds to identity', () => {
+  const x = bakeTiltTransform(400, 300, 360);
+  assert.equal(x.identity, true);
+  assert.deepEqual(x.mapPoint(10, 20), [10, 20]);
+});
+
+test('bakeTiltTransform: 90° CW maps centre→centre; corner into AABB canvas', () => {
+  const srcW = 200, srcH = 100;
+  const x = bakeTiltTransform(srcW, srcH, 90);
+  assert.equal(x.identity, false);
+  // Rotated AABB of 200×100 about centre ≈ 100×200 (ceil may add 1px from float).
+  assert.ok(x.outW >= 100 && x.outW <= 101, `outW=${x.outW}`);
+  assert.ok(x.outH >= 200 && x.outH <= 201, `outH=${x.outH}`);
+  const [cx, cy] = x.mapPoint(srcW / 2, srcH / 2);
+  assert.ok(Math.abs(cx - x.outW / 2) < 1e-9);
+  assert.ok(Math.abs(cy - x.outH / 2) < 1e-9);
+  // Top-left (0,0) relative (−100,−50) → CW90 → (~50,~−100) → +out/2
+  const [tlx, tly] = x.mapPoint(0, 0);
+  assert.ok(Math.abs(tlx - (50 + x.outW / 2)) < 1e-4, `tlx=${tlx}`);
+  assert.ok(Math.abs(tly - (-100 + x.outH / 2)) < 1e-4, `tly=${tly}`);
+});
+
+test('bakeTiltTransform: maps a tape-like vertical segment; direction rotates with tilt', () => {
+  const srcW = 400, srcH = 600;
+  const start: [number, number] = [200, 100];
+  const end: [number, number] = [200, 500];
+  const preUx = 0, preUy = 1; // vertical down
+  for (const tilt of [15, 30, -20, 45]) {
+    const x = bakeTiltTransform(srcW, srcH, tilt);
+    const a = x.mapPoint(start[0], start[1]);
+    const b = x.mapPoint(end[0], end[1]);
+    const dx = b[0] - a[0], dy = b[1] - a[1];
+    const len = Math.hypot(dx, dy) || 1;
+    const ux = dx / len, uy = dy / len;
+    // Expected: rotate (preUx, preUy) clockwise by tilt
+    const rad = (((tilt % 360) + 360) % 360) * Math.PI / 180;
+    const cos = Math.cos(rad), sin = Math.sin(rad);
+    const expUx = preUx * cos - preUy * sin;
+    const expUy = preUx * sin + preUy * cos;
+    const dot = ux * expUx + uy * expUy;
+    assert.ok(dot > 0.999, `tilt=${tilt} direction mismatch dot=${dot}`);
+  }
+});
diff --git a/frontend/src/png/rotateCal.ts b/frontend/src/png/rotateCal.ts
index 2c75311..31153b9 100644
--- a/frontend/src/png/rotateCal.ts
+++ b/frontend/src/png/rotateCal.ts
@@ -12,28 +12,43 @@ export interface RotatedCal {
   source: Pick<Source, 'widthPx' | 'heightPx'>;
 }
 
+/** Pure math for the same clockwise-about-centre bake used by rotateCalibration. */
+export interface BakeTiltTransform {
+  /** True when tilt is ~0° (identity; no resample). */
+  identity: boolean;
+  tiltDeg: number;
+  srcW: number;
+  srcH: number;
+  outW: number;
+  outH: number;
+  /** Map a point from pre-bake source coords into baked canvas coords. */
+  mapPoint: (x: number, y: number) => [number, number];
+}
+
 /**
- * Rotate source clockwise by tiltDeg (same sign as CSS/canvas rotate).
- * Returns a new bitmap the caller must close, plus recomputed mask geometry.
- * Identity (≈0°) returns clones of inputs without resampling.
+ * Clockwise rotation about image centre; output canvas sized to AABB of rotated
+ * corners — identical sizing to rotateCalibration (canvas translate+rotate path).
  */
-export async function rotateCalibration(
-  bitmap: ImageBitmap,
-  mask: Mask,
-  source: Pick<Source, 'widthPx' | 'heightPx'>,
+export function bakeTiltTransform(
+  srcW: number,
+  srcH: number,
   tiltDeg: number,
-): Promise<RotatedCal> {
+): BakeTiltTransform {
   const a = ((tiltDeg % 360) + 360) % 360;
   if (a < 0.05 || a > 359.95) {
-    // No meaningful rotation — hand back the same bitmap reference; caller closes once.
-    return { bitmap, mask, source };
+    return {
+      identity: true,
+      tiltDeg: a,
+      srcW,
+      srcH,
+      outW: srcW,
+      outH: srcH,
+      mapPoint: (x, y) => [x, y],
+    };
   }
-  const srcW = source.widthPx;
-  const srcH = source.heightPx;
   const rad = (a * Math.PI) / 180;
   const cos = Math.cos(rad);
   const sin = Math.sin(rad);
-  // Axis-aligned bounds of rotated rectangle corners.
   const corners = [
     [0, 0],
     [srcW, 0],
@@ -53,6 +68,39 @@ export async function rotateCalibration(
   }
   const outW = Math.max(1, Math.ceil(maxX - minX));
   const outH = Math.max(1, Math.ceil(maxY - minY));
+  return {
+    identity: false,
+    tiltDeg: a,
+    srcW,
+    srcH,
+    outW,
+    outH,
+    mapPoint: (x, y) => {
+      const cx = x - srcW / 2;
+      const cy = y - srcH / 2;
+      return [cx * cos - cy * sin + outW / 2, cx * sin + cy * cos + outH / 2];
+    },
+  };
+}
+
+/**
+ * Rotate source clockwise by tiltDeg (same sign as CSS/canvas rotate).
+ * Returns a new bitmap the caller must close, plus recomputed mask geometry.
+ * Identity (≈0°) returns clones of inputs without resampling.
+ */
+export async function rotateCalibration(
+  bitmap: ImageBitmap,
+  mask: Mask,
+  source: Pick<Source, 'widthPx' | 'heightPx'>,
+  tiltDeg: number,
+): Promise<RotatedCal> {
+  const xform = bakeTiltTransform(source.widthPx, source.heightPx, tiltDeg);
+  if (xform.identity) {
+    // No meaningful rotation — hand back the same bitmap reference; caller closes once.
+    return { bitmap, mask, source };
+  }
+  const { srcW, srcH, outW, outH, tiltDeg: a } = xform;
+  const rad = (a * Math.PI) / 180;
   const canvas = new OffscreenCanvas(outW, outH);
   const ctx = canvas.getContext('2d')!;
   ctx.translate(outW / 2, outH / 2);