diff --git a/docs/website-content.md b/docs/website-content.md
index 0f43433..43d8c9f 100644
--- a/docs/website-content.md
+++ b/docs/website-content.md
@@ -63,6 +63,11 @@ pages with `reproduces` and to depicted objects with `depicts`.
 The collection contains objects and sources. Its subnavigation uses the label
 “Documents and images” / “Documentos e imágenes” for sources. The site publishes
 object and source indexes and detail pages.
+The object index uses masonry placement in the shortest available column, with
+four columns on wider screens and two on narrow screens. Images retain their
+natural proportions without cropping or height limits. A small client script
+repacks the cards after search filtering and size changes; document and keyboard
+order remain unchanged. Without JavaScript, the same cards use a regular grid.
 Sources are listed at `/en/sources/` and `/es/fuentes/`; articles are listed
 at `/en/articles/` and `/es/articulos/`, labelled “Articles” / “Artículos”.
 Articles are authored MoSA publications rather than collection records. Their
diff --git a/src/components/collection-masonry.test.ts b/src/components/collection-masonry.test.ts
new file mode 100644
index 0000000..7a894a5
--- /dev/null
+++ b/src/components/collection-masonry.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import { masonryPositions } from "./collection-masonry";
+
+describe("collection masonry", () => {
+  it("packs short cards alongside an uncapped tall image in document order", () => {
+    expect(masonryPositions([2000, 100, 100, 50], 2, 20)).toEqual({
+      positions: [
+        { column: 0, top: 0 },
+        { column: 1, top: 0 },
+        { column: 1, top: 120 },
+        { column: 1, top: 240 },
+      ],
+      height: 2000,
+    });
+  });
+
+  it("starts again with the first column for a filtered result", () => {
+    expect(masonryPositions([50], 4, 20)).toEqual({
+      positions: [{ column: 0, top: 0 }],
+      height: 50,
+    });
+    expect(masonryPositions([], 4, 20)).toEqual({ positions: [], height: 0 });
+  });
+
+  it("breaks equal-height ties from left to right and omits the final gap", () => {
+    expect(masonryPositions([100, 100, 50, 60], 2, 20)).toEqual({
+      positions: [
+        { column: 0, top: 0 },
+        { column: 1, top: 0 },
+        { column: 0, top: 120 },
+        { column: 1, top: 120 },
+      ],
+      height: 180,
+    });
+  });
+});
diff --git a/src/components/collection-masonry.ts b/src/components/collection-masonry.ts
new file mode 100644
index 0000000..9c11e97
--- /dev/null
+++ b/src/components/collection-masonry.ts
@@ -0,0 +1,59 @@
+export function masonryPositions(heights: number[], columns: number, gap: number) {
+  const bottoms = Array<number>(columns).fill(0);
+  const positions = heights.map((height) => {
+    const top = Math.min(...bottoms);
+    const column = bottoms.indexOf(top);
+    bottoms[column] = top + height + gap;
+    return { column, top };
+  });
+  return { positions, height: Math.max(0, ...bottoms) - (heights.length ? gap : 0) };
+}
+
+export function initialiseCollectionMasonry(container: HTMLElement, cards: HTMLElement[]) {
+  container.dataset.masonry = "true";
+
+  const layout = () => {
+    if (container.hidden) return;
+    const style = getComputedStyle(container);
+    const columns = Number(style.getPropertyValue("--collection-columns"));
+    const columnGap = Number.parseFloat(style.columnGap);
+    const rowGap = Number.parseFloat(style.rowGap);
+    const width = container.getBoundingClientRect().width;
+    const columnWidth = (width - columnGap * (columns - 1)) / columns;
+    const visible = cards.filter((card) => !card.hidden);
+    // Read all sizes before writing positions to avoid repeated layout work.
+    const { positions, height } = masonryPositions(
+      visible.map((card) => card.getBoundingClientRect().height),
+      columns,
+      rowGap,
+    );
+    visible.forEach((card, index) => {
+      const { column, top } = positions[index];
+      card.style.left = `${column * (columnWidth + columnGap)}px`;
+      card.style.top = `${top}px`;
+    });
+    container.style.height = `${height}px`;
+  };
+
+  let frame = 0;
+  const scheduleLayout = () => {
+    cancelAnimationFrame(frame);
+    frame = requestAnimationFrame(layout);
+  };
+  let previousWidth = 0;
+  const observer = new ResizeObserver((entries) => {
+    const changed = entries.some((entry) => {
+      if (entry.target !== container) return true;
+      // Our own height updates must not trigger another layout indefinitely.
+      const width = entry.contentRect.width;
+      if (width === previousWidth) return false;
+      previousWidth = width;
+      return true;
+    });
+    if (changed) scheduleLayout();
+  });
+  observer.observe(container);
+  for (const card of cards) observer.observe(card);
+  layout();
+  return layout;
+}
diff --git a/src/i18n/collection-client.ts b/src/i18n/collection-client.ts
index 7c2d015..86aa22b 100644
--- a/src/i18n/collection-client.ts
+++ b/src/i18n/collection-client.ts
@@ -1,3 +1,4 @@
+import { initialiseCollectionMasonry } from "../components/collection-masonry";
 import { referenceCount } from "./messages";
 import { matchesSearch } from "./search";
 import { updateLanguageLinks } from "./switcher";
@@ -11,6 +12,7 @@ const cards = Array.from(document.querySelectorAll<HTMLElement>(".collection-car
 const locale =
   document.querySelector<HTMLElement>("[data-locale]")?.dataset.locale === "en" ? "en" : "es";
 if (controls && search && results && count && empty && cards.length) {
+  const layout = initialiseCollectionMasonry(results, cards);
   controls.hidden = false;
   const filter = () => {
     let visible = 0;
@@ -26,6 +28,7 @@ if (controls && search && results && count && empty && cards.length) {
     count.textContent = referenceCount(locale, { count: visible });
     empty.hidden = visible > 0;
     results.hidden = visible === 0;
+    layout();
     const url = new URL(location.href);
     for (const key of ["concept", "type", "view"]) url.searchParams.delete(key);
     if (search.value) url.searchParams.set("q", search.value);
diff --git a/src/page-templates/CollectionPage.astro b/src/page-templates/CollectionPage.astro
index 825496f..35b2d4d 100644
--- a/src/page-templates/CollectionPage.astro
+++ b/src/page-templates/CollectionPage.astro
@@ -86,16 +86,17 @@ const t = messages(locale);
   margin-bottom: 2rem;
 }
 .collection-grid {
+  --collection-columns: 4;
+  --collection-column-gap: 1rem;
   display: grid;
-  grid-template-columns: repeat(4, minmax(0, 1fr));
-  gap: 3.5rem 1rem;
+  grid-template-columns: repeat(var(--collection-columns), minmax(0, 1fr));
+  align-items: start;
+  gap: 3.5rem var(--collection-column-gap);
   margin-top: 2rem;
 }
-.collection-card:nth-child(3) {
-  grid-column: 4;
-}
-.collection-card:nth-child(5) {
-  grid-column: 3;
+.collection-grid[data-masonry] {
+  display: block;
+  position: relative;
 }
 .collection-caption {
   margin-top: 1.25rem;
@@ -128,15 +129,11 @@ const t = messages(locale);
   margin-right: 0.5rem;
   text-decoration: underline;
 }
-
-@media (min-width: 801px) {
-  .collection-grid:has(.collection-card[hidden]) {
-    grid-template-columns: repeat(4, minmax(0, 1fr));
-  }
-  .collection-grid:has(.collection-card[hidden]) .collection-card {
-    grid-column: auto;
-  }
+.collection-grid[data-masonry] .collection-card {
+  position: absolute;
+  width: calc((100% - (var(--collection-columns) - 1) * var(--collection-column-gap)) / var(--collection-columns));
 }
+
 @media (max-width: 1100px) {
   .info-card {
     padding: 1.5rem;
@@ -144,11 +141,8 @@ const t = messages(locale);
 }
 @media (max-width: 800px) {
   .collection-grid {
-    grid-template-columns: repeat(2, minmax(0, 1fr));
-    gap: 2.5rem 1rem;
-  }
-  .collection-card:nth-child(n) {
-    grid-column: auto;
+    --collection-columns: 2;
+    row-gap: 2.5rem;
   }
 }
 @media (max-width: 520px) {