diff --git a/public/js/webgl-book-shape-lab.js b/public/js/webgl-book-shape-lab.js
index c241e32..29c968d 100644
--- a/public/js/webgl-book-shape-lab.js
+++ b/public/js/webgl-book-shape-lab.js
@@ -6,6 +6,9 @@ const progressInput = document.getElementById('progress');
 const progressValue = document.getElementById('progress_value');
 const pageCountInput = document.getElementById('page_count');
 const pageCountValue = document.getElementById('page_count_value');
+const flipBackwardButton = document.getElementById('flip_backward');
+const flipForwardButton = document.getElementById('flip_forward');
+const flipCountValue = document.getElementById('flip_count');
 const urlParams = new URLSearchParams(window.location.search);
 
 const renderer = new THREE.WebGLRenderer({ canvas, antialias: true });
@@ -42,6 +45,7 @@ const materials = {
     pagesLeft: new THREE.MeshBasicMaterial({ color: 0xd8c7a4, side: THREE.DoubleSide }),
     pagesRight: new THREE.MeshBasicMaterial({ color: 0xe7d6b4, side: THREE.DoubleSide }),
     topPage: new THREE.MeshBasicMaterial({ color: 0xf1dfba, side: THREE.DoubleSide }),
+    flippingPage: new THREE.MeshBasicMaterial({ color: 0xf3dfb6, side: THREE.DoubleSide }),
     edge: new THREE.MeshBasicMaterial({ color: 0xb99a68, side: THREE.DoubleSide }),
     hinge: new THREE.MeshBasicMaterial({ color: 0x2b0808 })
 };
@@ -59,10 +63,15 @@ let readingProgress = readInitialProgress();
 let pageCount = readInitialPageCount();
 let lastLengthError = 0;
 let lastSpacingError = 0;
+let lastBookModel = null;
+let activeFlip = null;
+let activeFlipMesh = null;
+let pendingPageFlips = 0;
 progressInput.value = readingProgress.toFixed(3);
 progressValue.value = readingProgress.toFixed(2);
 pageCountInput.value = String(pageCount);
 pageCountValue.value = String(pageCount);
+updateFlipControls();
 rebuildBook();
 resize();
 animate();
@@ -75,6 +84,14 @@ pageCountInput.addEventListener('input', () => {
     setPageCount(pageCountInput.value);
 });
 
+flipBackwardButton.addEventListener('click', () => {
+    startPageFlip(-1);
+});
+
+flipForwardButton.addEventListener('click', () => {
+    startPageFlip(1);
+});
+
 window.addEventListener('resize', resize);
 
 window.BookShapeLab = {
@@ -97,6 +114,12 @@ window.BookShapeLab = {
     setPageCount(value) {
         setPageCount(value);
         return pageCount;
+    },
+    flipForward() {
+        return startPageFlip(1);
+    },
+    flipBackward() {
+        return startPageFlip(-1);
     }
 };
 
@@ -118,19 +141,24 @@ function snapPageCount(value) {
 function setReadingProgress(value) {
     const next = THREE.MathUtils.clamp(Number.parseFloat(value), 0, 1);
     if (!Number.isFinite(next)) return;
+    clearActiveFlip();
     readingProgress = next;
     progressInput.value = readingProgress.toFixed(3);
     progressValue.value = readingProgress.toFixed(2);
     rebuildBook();
+    updateFlipControls();
 }
 
 function setPageCount(value) {
     const next = snapPageCount(Number.parseFloat(value));
     if (!Number.isFinite(next)) return;
+    clearActiveFlip();
+    pendingPageFlips = 0;
     pageCount = next;
     pageCountInput.value = String(pageCount);
     pageCountValue.value = String(pageCount);
     rebuildBook();
+    updateFlipControls();
 }
 
 function rebuildBook() {
@@ -145,11 +173,13 @@ function rebuildBook() {
     const lines = simulatePageLines(bundleCount, pageWidth, spineWidth);
     lastLengthError = measureLineLengthError(lines, pageWidth);
     lastSpacingError = measureStackSpacingError(lines);
+    lastBookModel = { coverDepth, pageWidth, pageDepth, bundleCount, spineWidth, lines };
 
     addCoverAssembly(pageWidth, coverDepth, coverThickness, spineWidth);
     addClothSpine(pageDepth, spineWidth);
     addSimulatedStackBodies(lines, pageDepth);
     addSimulatedPageLines(lines, pageDepth);
+    updateFlipControls();
 }
 
 function clearGroup(group) {
@@ -749,6 +779,261 @@ function createEndpointPolyline(lines, depth) {
     return new THREE.BufferGeometry().setFromPoints(points);
 }
 
+function startPageFlip(direction) {
+    if (activeFlip || !lastBookModel || !canPageFlip(direction)) return false;
+    const sourceSide = direction > 0 ? 1 : -1;
+    const sourceLine = topVisibleLine(sourceSide);
+    const destinationLine = topVisibleLine(-sourceSide);
+    if (!sourceLine || !destinationLine) return false;
+
+    activeFlip = {
+        direction,
+        sourceLine,
+        destinationLine,
+        startTime: performance.now(),
+        duration: 1800
+    };
+    updateFlipControls();
+    updateActiveFlip(activeFlip.startTime);
+    return true;
+}
+
+function canPageFlip(direction) {
+    if (!lastBookModel) return false;
+    if (direction > 0) return readingProgress < 1;
+    return readingProgress > 0;
+}
+
+function topVisibleLine(side) {
+    const sideLines = lastBookModel.lines
+        .filter((line) => line.side === side)
+        .sort((a, b) => side < 0 ? a.t - b.t : b.t - a.t);
+    return sideLines[sideLines.length - 1] ?? null;
+}
+
+function updateActiveFlip(now) {
+    if (!activeFlip || !lastBookModel) return;
+    const elapsed = (now - activeFlip.startTime) / activeFlip.duration;
+    const t = THREE.MathUtils.clamp(elapsed, 0, 1);
+    const surface = buildFlippingPageSurface(activeFlip.sourceLine, activeFlip.destinationLine, activeFlip.direction, easeInOutCubic(t));
+    setActivePageGeometry(surface);
+    if (t < 1) return;
+
+    finishActiveFlip();
+}
+
+function buildFlippingPageSurface(sourceLine, destinationLine, direction, t) {
+    const widthSegments = sourceLine.points.length - 1;
+    const depthSegments = 18;
+    const zFront = lastBookModel.pageDepth * 0.5 + 0.018;
+    const zBack = -lastBookModel.pageDepth * 0.5 - 0.018;
+    if (t <= 0) return createRestingPageSurface(sourceLine.points, depthSegments, zFront, zBack);
+    if (t >= 1) return createRestingPageSurface(destinationLine.points, depthSegments, zFront, zBack);
+
+    const anchorT = THREE.MathUtils.lerp(sourceLine.t, destinationLine.t, t);
+    const anchor = spineCurvePoint(anchorT, lastBookModel.spineWidth);
+    const sourceSide = direction > 0 ? 1 : -1;
+    const startAngle = sourceSide > 0 ? 0 : Math.PI;
+    const baseAngle = startAngle + direction * Math.PI * t;
+    const lift = Math.sin(Math.PI * t);
+    const curlStrength = direction * 0.48 * lift;
+    const surface = [];
+    for (let widthIndex = 0; widthIndex <= widthSegments; widthIndex += 1) {
+        const u = widthIndex / widthSegments;
+        const radius = lastBookModel.pageWidth * u;
+        const row = [];
+        for (let depthIndex = 0; depthIndex <= depthSegments; depthIndex += 1) {
+            const v = depthIndex / depthSegments;
+            const z = THREE.MathUtils.lerp(zFront, zBack, v);
+            const depthWave = (v - 0.5) * 0.22 * lift * (0.15 + u * 0.85);
+            const curl = curlStrength * Math.sin(Math.PI * u) + direction * depthWave;
+            const angle = baseAngle + curl;
+            const stackPoint = interpolatePagePoint(sourceLine.points, destinationLine.points, widthIndex, t);
+            const relaxedY = THREE.MathUtils.lerp(stackPoint.y, anchor.y + Math.sin(angle) * radius, lift);
+            const point = {
+                x: anchor.x + Math.cos(angle) * radius,
+                y: relaxedY + 0.055 * lift * Math.sin(Math.PI * u),
+                z
+            };
+            keepFlippingSurfacePointAboveStacks(point, lift);
+            row.push(point);
+        }
+        surface.push(row);
+    }
+    return surface;
+}
+
+function createRestingPageSurface(points, depthSegments, zFront, zBack) {
+    return points.map((point) => {
+        const row = [];
+        for (let depthIndex = 0; depthIndex <= depthSegments; depthIndex += 1) {
+            row.push({
+                x: point.x,
+                y: point.y,
+                z: THREE.MathUtils.lerp(zFront, zBack, depthIndex / depthSegments)
+            });
+        }
+        return row;
+    });
+}
+
+function interpolatePagePoint(sourcePoints, destinationPoints, index, t) {
+    const source = sourcePoints[index];
+    const destination = destinationPoints[index];
+    return {
+        x: THREE.MathUtils.lerp(source.x, destination.x, t),
+        y: THREE.MathUtils.lerp(source.y, destination.y, t)
+    };
+}
+
+function keepFlippingSurfacePointAboveStacks(point, lift) {
+    const envelopeY = stackEnvelopeYAtX(point.x);
+    if (envelopeY === null) return;
+    const clearance = 0.016 + lift * 0.045;
+    point.y = Math.max(point.y, envelopeY + clearance);
+}
+
+function keepFlippingPageAboveStacks(points, lift) {
+    for (let index = 1; index < points.length; index += 1) {
+        const u = index / (points.length - 1);
+        const envelopeY = stackEnvelopeYAtX(points[index].x);
+        if (envelopeY === null) continue;
+        const clearance = 0.018 + lift * (0.05 + 0.05 * Math.sin(Math.PI * u));
+        points[index].y = Math.max(points[index].y, envelopeY + clearance);
+    }
+}
+
+function stackEnvelopeYAtX(x) {
+    let envelope = null;
+    lastBookModel.lines.forEach((line) => {
+        const y = lineYAtX(line.points, x);
+        if (y === null) return;
+        envelope = envelope === null ? y : Math.max(envelope, y);
+    });
+    return envelope;
+}
+
+function lineYAtX(points, x) {
+    let y = null;
+    for (let index = 0; index < points.length - 1; index += 1) {
+        const a = points[index];
+        const b = points[index + 1];
+        const minX = Math.min(a.x, b.x) - 0.00001;
+        const maxX = Math.max(a.x, b.x) + 0.00001;
+        if (x < minX || x > maxX) continue;
+        const span = b.x - a.x;
+        const segmentY = Math.abs(span) < 0.00001
+            ? Math.max(a.y, b.y)
+            : THREE.MathUtils.lerp(a.y, b.y, (x - a.x) / span);
+        y = y === null ? segmentY : Math.max(y, segmentY);
+    }
+    return y;
+}
+
+function setActivePageGeometry(surface) {
+    const geometry = createFlippingPageGeometry(surface);
+    if (!activeFlipMesh) {
+        activeFlipMesh = new THREE.Mesh(geometry, materials.flippingPage);
+        book.add(activeFlipMesh);
+        return;
+    }
+    activeFlipMesh.geometry.dispose();
+    activeFlipMesh.geometry = geometry;
+}
+
+function createFlippingPageGeometry(surface) {
+    const positions = [];
+    const indices = [];
+    const topGrid = [];
+    const bottomGrid = [];
+    const pageThickness = 0.006;
+    const widthSegments = surface.length - 1;
+    const depthSegments = surface[0].length - 1;
+    const push = (point, yOffset) => {
+        const index = positions.length / 3;
+        positions.push(point.x, point.y + yOffset, point.z);
+        return index;
+    };
+
+    surface.forEach((rowPoints) => {
+        const topRow = [];
+        const bottomRow = [];
+        rowPoints.forEach((point) => {
+            topRow.push(push(point, pageThickness));
+            bottomRow.push(push(point, 0));
+        });
+        topGrid.push(topRow);
+        bottomGrid.push(bottomRow);
+    });
+    for (let index = 0; index < widthSegments; index += 1) {
+        for (let zIndex = 0; zIndex < depthSegments; zIndex += 1) {
+            const a = topGrid[index][zIndex];
+            const b = topGrid[index + 1][zIndex];
+            const c = topGrid[index][zIndex + 1];
+            const d = topGrid[index + 1][zIndex + 1];
+            const bottomA = bottomGrid[index][zIndex];
+            const bottomB = bottomGrid[index + 1][zIndex];
+            const bottomC = bottomGrid[index][zIndex + 1];
+            const bottomD = bottomGrid[index + 1][zIndex + 1];
+            indices.push(a, c, b);
+            indices.push(b, c, d);
+            indices.push(bottomA, bottomB, bottomC);
+            indices.push(bottomB, bottomD, bottomC);
+        }
+    }
+    for (let index = 0; index < widthSegments; index += 1) {
+        addWall(topGrid[index][0], topGrid[index + 1][0], bottomGrid[index][0], bottomGrid[index + 1][0]);
+        addWall(topGrid[index][depthSegments], topGrid[index + 1][depthSegments], bottomGrid[index][depthSegments], bottomGrid[index + 1][depthSegments]);
+    }
+    for (let zIndex = 0; zIndex < depthSegments; zIndex += 1) {
+        addWall(topGrid[0][zIndex], topGrid[0][zIndex + 1], bottomGrid[0][zIndex], bottomGrid[0][zIndex + 1]);
+        addWall(topGrid[widthSegments][zIndex], topGrid[widthSegments][zIndex + 1], bottomGrid[widthSegments][zIndex], bottomGrid[widthSegments][zIndex + 1]);
+    }
+
+    const geometry = new THREE.BufferGeometry();
+    geometry.setIndex(indices);
+    geometry.setAttribute('position', new THREE.Float32BufferAttribute(positions, 3));
+    geometry.computeVertexNormals();
+    return geometry;
+
+    function addWall(topA, topB, bottomA, bottomB) {
+        indices.push(topA, bottomA, topB);
+        indices.push(topB, bottomA, bottomB);
+    }
+}
+
+function finishActiveFlip() {
+    const direction = activeFlip.direction;
+    clearActiveFlip();
+    pendingPageFlips += direction;
+    if (Math.abs(pendingPageFlips) >= 10) {
+        const commitDirection = Math.sign(pendingPageFlips);
+        pendingPageFlips -= commitDirection * 10;
+        const step = 1 / (lastBookModel.bundleCount - 1);
+        setReadingProgress(readingProgress + commitDirection * step);
+        return;
+    }
+    updateFlipControls();
+}
+
+function clearActiveFlip() {
+    activeFlip = null;
+    if (!activeFlipMesh) return;
+    book.remove(activeFlipMesh);
+    activeFlipMesh.geometry.dispose();
+    activeFlipMesh = null;
+}
+
+function updateFlipControls() {
+    flipBackwardButton.disabled = Boolean(activeFlip) || !canPageFlip(-1);
+    flipForwardButton.disabled = Boolean(activeFlip) || !canPageFlip(1);
+    flipCountValue.textContent = `${Math.abs(pendingPageFlips)} / 10`;
+}
+
+function easeInOutCubic(t) {
+    return t < 0.5 ? 4 * t * t * t : 1 - Math.pow(-2 * t + 2, 3) * 0.5;
+}
+
 function resize() {
     const width = window.innerWidth;
     const height = window.innerHeight;
@@ -763,6 +1048,7 @@ function animate() {
         const t = performance.now() * 0.00035;
         setReadingProgress(0.5 + Math.sin(t) * 0.48);
     }
+    updateActiveFlip(performance.now());
     controls.update();
     renderer.render(scene, camera);
 }
diff --git a/public/webgl-book-shape-lab.html b/public/webgl-book-shape-lab.html
index 5a1ce00..fd30fa5 100644
--- a/public/webgl-book-shape-lab.html
+++ b/public/webgl-book-shape-lab.html
@@ -42,6 +42,21 @@
         #page_count {
             width: 100%;
         }
+
+        button {
+            min-height: 32px;
+            border: 1px solid rgba(255, 255, 255, 0.25);
+            border-radius: 4px;
+            background: #2b2d30;
+            color: #f0f0f0;
+            font: inherit;
+            cursor: pointer;
+        }
+
+        button:disabled {
+            cursor: default;
+            opacity: 0.45;
+        }
     </style>
 </head>
 <body>
@@ -53,7 +68,10 @@
         <label for="page_count">Book pages</label>
         <input id="page_count" type="range" min="40" max="600" step="10" value="240">
         <output id="page_count_value" for="page_count">240</output>
+        <button id="flip_backward" type="button">Backward</button>
+        <button id="flip_forward" type="button">Forward</button>
+        <output id="flip_count">0 / 10</output>
     </div>
-    <script type="module" src="/js/webgl-book-shape-lab.js"></script>
+    <script type="module" src="/js/webgl-book-shape-lab.js?v=flip-3d-surface-3"></script>
 </body>
 </html>