diff --git a/web/canvas.ts b/web/canvas.ts
index 481b0af..6ab8f1d 100644
--- a/web/canvas.ts
+++ b/web/canvas.ts
@@ -55,6 +55,8 @@ export type CanvasComponent = {
   kind: string;
   /** The heading the page draws above the block, or null. Every kind may have one. */
   title?: string | null;
+  /** `main`, `page` or null — how wide it is allowed to be. See the site's schema. */
+  width?: string | null;
   /*
     One optional field per kind that the panel reads back. Not `Record<string, unknown>`: every reader
     would then need a cast, and the point of naming them is that the compiler knows `free-text` holds
diff --git a/web/sections/BlockSettings.tsx b/web/sections/BlockSettings.tsx
index 9788aaa..bc55b2f 100644
--- a/web/sections/BlockSettings.tsx
+++ b/web/sections/BlockSettings.tsx
@@ -27,7 +27,12 @@ export const BlockSettings = ({
 }: {
   pageId: number;
   sectionId: number;
-  block: { id: number; title?: string | null; padding?: Record<string, { x?: number; y?: number }> };
+  block: {
+    id: number;
+    title?: string | null;
+    width?: string | null;
+    padding?: Record<string, { x?: number; y?: number }>;
+  };
   /** The padding scale, sent by the API so this file holds no copy of it. */
   steps: number[];
   onSaved: () => void;
@@ -39,13 +44,20 @@ export const BlockSettings = ({
   const [device] = useDevice();
   const breakpoint = ({ desktop: 'lg', tablet: 'md', mobile: 'base' } as const)[device];
 
-  const [draft, setDraft] = useState({ title: block.title ?? '', padding: block.padding ?? {} });
+  const [draft, setDraft] = useState({
+    title: block.title ?? '',
+    width: block.width ?? '',
+    padding: block.padding ?? {},
+  });
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [saved, flashSaved] = useFlash();
 
   // Keyed on the id, so re-reading the same block after a save does not throw away what is being typed.
-  useEffect(() => setDraft({ title: block.title ?? '', padding: block.padding ?? {} }), [block.id]);
+  useEffect(
+    () => setDraft({ title: block.title ?? '', width: block.width ?? '', padding: block.padding ?? {} }),
+    [block.id],
+  );
 
   /** What an axis falls back to at this width: the nearest smaller one that has a value. */
   const inherited = (axis: 'x' | 'y'): number | null => {
@@ -78,7 +90,7 @@ export const BlockSettings = ({
       // Empty is sent as an empty string, which the API reads as "take the heading off".
       await clientRef.current.patch(
         `/fofos-admin/pages/${pageId}/sections/${sectionId}/components/${block.id}`,
-        { title: draft.title, padding: draft.padding },
+        { title: draft.title, width: draft.width || null, padding: draft.padding },
       );
       flashSaved();
       onSaved();
@@ -89,7 +101,11 @@ export const BlockSettings = ({
     }
   };
 
-  const stored = JSON.stringify({ title: block.title ?? '', padding: block.padding ?? {} });
+  const stored = JSON.stringify({
+    title: block.title ?? '',
+    width: block.width ?? '',
+    padding: block.padding ?? {},
+  });
   const pending = JSON.stringify(draft);
 
   useAutosave(pending !== stored ? pending : null, busy, save);
@@ -108,6 +124,29 @@ export const BlockSettings = ({
         }}
       />
 
+      {/*
+        How wide it is allowed to be.
+
+        It was a NEGATIVE horizontal padding, which worked and asked the wrong thing: you had to know
+        the band's gutter and type its opposite, and it broke quietly the day somebody changed the
+        gutter. These two say what is wanted, and the page works out the number.
+      */}
+      <div className="mt-3 flex flex-col gap-1">
+        <label className="text-xs">Width</label>
+        <select
+          value={draft.width}
+          onChange={(event) => {
+            const value = event.currentTarget.value;
+            setDraft((was) => ({ ...was, width: value }));
+          }}
+          className="h-8 rounded-md border border-input bg-transparent px-2 text-xs"
+        >
+          <option value="">Inside the band</option>
+          <option value="main">Whole width of main</option>
+          <option value="page">Whole page width</option>
+        </select>
+      </div>
+
       {/*
         The block's own spacing, at the width the toggle above is showing.
 
@@ -139,20 +178,7 @@ export const BlockSettings = ({
                 <option value="">
                   {breakpoint === 'base' ? 'None' : from === null ? 'Inherit (none)' : `Inherit (${from})`}
                 </option>
-                {/*
-                  Horizontal offers NEGATIVES, vertical does not.
-
-                  A negative horizontal is how a block escapes the gutter its band applied — a video
-                  that runs to the edge of the window inside an inset band. It becomes `-mx-*`, which
-                  cancels the band's `px-*` exactly, so the value to pick is the band's own number.
-
-                  Vertically there is nothing to cancel: a band's vertical padding is the room around
-                  the whole band, and a block pulling itself into the one above is a bug.
-                */}
-                {(axis === 'x'
-                  ? [...steps.filter((step) => step > 0).map((step) => -step).reverse(), ...steps]
-                  : steps
-                ).map((step) => (
+                {steps.map((step) => (
                   <option key={step} value={String(step)}>
                     {step}
                   </option>