diff --git a/src/components/MediaMuteAndSwitchButton.module.css b/src/components/MediaMuteAndSwitchButton.module.css
index 092cb23ce..b58655ea6 100644
--- a/src/components/MediaMuteAndSwitchButton.module.css
+++ b/src/components/MediaMuteAndSwitchButton.module.css
@@ -157,8 +157,9 @@ Please see LICENSE in the repository root for full details.
  * The Background effects grid.
  *
  * Three across matches the design and keeps a thumbnail readable at a glance;
- * with no effect, blur, two shipped images and the add tile it also settles
- * into a tidy three-by-three at the maximum number of images.
+ * with no effect, blur and two shipped images, the five a user may add fill it
+ * to a whole three-by-three at the limit, where the add tile goes away, and
+ * four and the add tile fill it one short of it.
  *
  * minmax(0, 1fr) rather than 1fr: a grid track will not shrink below its
  * content's minimum width by default, and a tile carrying a label is wider
diff --git a/src/livekit/backgroundEffects.test.ts b/src/livekit/backgroundEffects.test.ts
index 6ad7de07c..d086cb890 100644
--- a/src/livekit/backgroundEffects.test.ts
+++ b/src/livekit/backgroundEffects.test.ts
@@ -5,6 +5,7 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
+import { maxAddedBackgrounds } from "./backgroundImages";
 import { describe, expect, test } from "vitest";
 
 import {
@@ -46,6 +47,15 @@ describe("the chosen background effect", () => {
     }
   });
 
+  // Three to a row, and the add tile goes at the limit. At four of their own
+  // the grid stopped at eight and left its last corner empty; the rule is that
+  // it is whole both at the limit and one short of it, with the add tile.
+  test("fills the grid at the limit and one short of it", () => {
+    const fixed = 2 + shippedBackgrounds.length; // no effect, blur, shipped
+    expect((fixed + maxAddedBackgrounds) % 3).toBe(0);
+    expect((fixed + maxAddedBackgrounds - 1 + 1) % 3).toBe(0);
+  });
+
   test("gives every shipped background an image to draw", () => {
     for (const background of shippedBackgrounds)
       expect(imagePathFor(background.id)).toBeTruthy();
diff --git a/src/livekit/backgroundImages.ts b/src/livekit/backgroundImages.ts
index 3ab2a1c5d..9d714e2d6 100644
--- a/src/livekit/backgroundImages.ts
+++ b/src/livekit/backgroundImages.ts
@@ -7,8 +7,15 @@ Please see LICENSE in the repository root for full details.
 
 import { logger } from "matrix-js-sdk/lib/logger";
 
-/** The most backgrounds of their own a device keeps. */
-export const maxAddedBackgrounds = 4;
+/**
+ * The most backgrounds of their own a device keeps.
+ *
+ * Five, because it fills the grid. With no effect, blur and the two shipped
+ * images that makes nine tiles at the limit, where the add tile goes; and one
+ * short of it, four and the add tile make nine too. At four the grid stopped
+ * at eight and left its last corner empty.
+ */
+export const maxAddedBackgrounds = 5;
 
 /**
  * The longest edge an added background is kept at.