diff --git a/src/layouts/BaseLayout.astro b/src/layouts/BaseLayout.astro
index c6491f0..c7dfb00 100644
--- a/src/layouts/BaseLayout.astro
+++ b/src/layouts/BaseLayout.astro
@@ -87,9 +87,6 @@ const base = import.meta.env.BASE_URL;
   }
 
   main {
-    max-width: var(--width-container);
-    margin-left: auto;
-    margin-right: auto;
     padding-left: var(--gutter);
     padding-right: var(--gutter);
   }
diff --git a/src/pages/posts/[...slug].astro b/src/pages/posts/[...slug].astro
index 3ff1d24..22e2232 100644
--- a/src/pages/posts/[...slug].astro
+++ b/src/pages/posts/[...slug].astro
@@ -41,7 +41,6 @@ const readingTime = post.body ? getReadingTime(post.body) : undefined;
 
 <style>
   .prose-wrapper {
-    max-width: var(--width-prose);
     margin-left: auto;
     margin-right: auto;
     padding-bottom: var(--space-12);
diff --git a/src/styles/prose.css b/src/styles/prose.css
index dee6f77..cc5144c 100644
--- a/src/styles/prose.css
+++ b/src/styles/prose.css
@@ -1,5 +1,34 @@
 /* Prose — typographic treatment for rendered markdown */
 
+/* --- Three-width layout system --- */
+/* Default: all direct children constrained to reading width */
+
+.prose > * {
+  max-width: var(--width-prose);
+  margin-left: auto;
+  margin-right: auto;
+}
+
+/* Wide: code blocks, SVGs, figures, and .wide/.breakout elements */
+
+.prose > pre,
+.prose > svg,
+.prose > figure,
+.prose > .wide,
+.prose > .breakout {
+  max-width: var(--width-code);
+  margin-left: auto;
+  margin-right: auto;
+}
+
+/* Full-bleed: hero diagrams, comparison layouts */
+
+.prose > .full {
+  max-width: var(--width-container);
+  margin-left: auto;
+  margin-right: auto;
+}
+
 /* --- Headings (Structure voice: Inter) --- */
 
 .prose h2 {
@@ -107,14 +136,9 @@
   color: inherit;
 }
 
-/* --- Code blocks (breakout width) --- */
+/* --- Code blocks (wide breakout via three-width system) --- */
 
 .prose pre {
-  position: relative;
-  left: 50%;
-  transform: translateX(-50%);
-  width: min(var(--width-code), calc(100vw - var(--gutter) * 2));
-  max-width: var(--width-code);
   border: 1px solid var(--border-subtle);
   border-left: 3px solid var(--accent-primary);
   border-radius: var(--rounded-md);
@@ -152,6 +176,16 @@
   outline-offset: 2px;
 }
 
+/* --- Inline SVG diagrams --- */
+
+.prose svg {
+  width: 100%;
+  height: auto;
+  border-radius: var(--rounded-md);
+  margin-top: var(--space-5);
+  margin-bottom: var(--space-5);
+}
+
 /* --- Horizontal rules --- */
 
 .prose hr {
@@ -203,4 +237,4 @@
 .prose td {
   padding: var(--space-1) var(--space-2);
   border-bottom: 1px solid var(--border-subtle);
-}
+}
\ No newline at end of file
diff --git a/tests/final-verification.test.mjs b/tests/final-verification.test.mjs
index e094974..25acd18 100644
--- a/tests/final-verification.test.mjs
+++ b/tests/final-verification.test.mjs
@@ -231,10 +231,15 @@ describe('Responsive behavior', () => {
       'Title should use smaller type scale at breakpoints');
   });
 
-  test('code blocks respect gutters on mobile', () => {
+  test('code blocks respect gutters via three-width layout system', () => {
     const prose = readFileSync('src/styles/prose.css', 'utf-8');
-    assert.ok(prose.includes('100vw') && prose.includes('--gutter'),
-      'Code blocks should constrain to viewport minus gutters');
+    const layout = readFileSync('src/layouts/BaseLayout.astro', 'utf-8');
+    // Code blocks self-constrain via max-width in the breakout pattern
+    assert.ok(prose.includes('--width-code'),
+      'Code blocks should use --width-code for wide breakout');
+    // Gutters are provided by main element padding
+    assert.ok(layout.includes('--gutter'),
+      'Main element should provide gutter padding for responsive safety');
   });
 });
 
diff --git a/tests/post-page.test.mjs b/tests/post-page.test.mjs
index f03f27c..4d2baf5 100644
--- a/tests/post-page.test.mjs
+++ b/tests/post-page.test.mjs
@@ -126,10 +126,9 @@ describe('Post page template', () => {
   });
 
   describe('Style', () => {
-    it('should have .prose-wrapper with prose max-width, auto margins, and space-12 bottom padding', () => {
+    it('should have .prose-wrapper with auto margins and space-12 bottom padding (no max-width — children self-constrain)', () => {
       const content = readPage();
       assert.match(content, /\.prose-wrapper/, 'should style .prose-wrapper');
-      assert.match(content, /max-width\s*:\s*var\(--width-prose\)/, 'should have prose max-width');
       assert.match(content, /margin-left\s*:\s*auto/, 'should have auto left margin');
       assert.match(content, /margin-right\s*:\s*auto/, 'should have auto right margin');
       assert.match(content, /padding-bottom\s*:\s*var\(--space-12\)/, 'should have space-12 bottom padding');
diff --git a/tests/three-width-layout.test.mjs b/tests/three-width-layout.test.mjs
new file mode 100644
index 0000000..7179323
--- /dev/null
+++ b/tests/three-width-layout.test.mjs
@@ -0,0 +1,119 @@
+import { describe, it } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { readFileSync } from 'node:fs';
+
+const prosePath = 'src/styles/prose.css';
+const layoutPath = 'src/layouts/BaseLayout.astro';
+const postTemplatePath = 'src/pages/posts/[...slug].astro';
+
+function readProse() {
+  return readFileSync(prosePath, 'utf-8');
+}
+
+function readLayout() {
+  return readFileSync(layoutPath, 'utf-8');
+}
+
+function readPostTemplate() {
+  return readFileSync(postTemplatePath, 'utf-8');
+}
+
+describe('Three-width layout system', () => {
+  describe('Reading width (default)', () => {
+    it('should constrain all direct children of .prose to reading width by default', () => {
+      const css = readProse();
+      // .prose > * should have max-width: var(--width-prose)
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*max-width:\s*var\(--width-prose\)/s,
+        '.prose > * should set max-width to --width-prose');
+    });
+
+    it('should center prose children with auto margins', () => {
+      const css = readProse();
+      // .prose > * should have margin-left: auto and margin-right: auto
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*margin-left:\s*auto/s,
+        '.prose > * should have margin-left: auto');
+      assert.match(css, /\.prose\s*>\s*\*\s*\{[^}]*margin-right:\s*auto/s,
+        '.prose > * should have margin-right: auto');
+    });
+  });
+
+  describe('Wide width (code blocks, SVGs, figures)', () => {
+    it('should break code blocks to wide width', () => {
+      const css = readProse();
+      // .prose > pre should have max-width: var(--width-code)
+      assert.match(css, /\.prose\s*>\s*pre[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > pre should set max-width to --width-code');
+    });
+
+    it('should break SVGs to wide width', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*svg[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > svg should set max-width to --width-code');
+    });
+
+    it('should break .breakout and .wide elements to wide width', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*\.wide[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > .wide should set max-width to --width-code');
+      assert.match(css, /\.prose\s*>\s*\.breakout[^{]*\{[^}]*max-width:\s*var\(--width-code\)/s,
+        '.prose > .breakout should set max-width to --width-code');
+    });
+  });
+
+  describe('Full width (full-bleed)', () => {
+    it('should support full-bleed elements with .full class', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s*>\s*\.full[^{]*\{[^}]*max-width:\s*var\(--width-container\)/s,
+        '.prose > .full should set max-width to --width-container');
+    });
+  });
+
+  describe('SVG styling', () => {
+    it('should make SVGs responsive with width 100% and auto height', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*width:\s*100%/s,
+        'SVGs should have width: 100%');
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*height:\s*auto/s,
+        'SVGs should have height: auto');
+    });
+
+    it('should give SVGs proper vertical spacing', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*margin-top:\s*var\(--space-5\)/s,
+        'SVGs should have --space-5 top margin');
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*margin-bottom:\s*var\(--space-5\)/s,
+        'SVGs should have --space-5 bottom margin');
+    });
+
+    it('should give SVGs rounded corners', () => {
+      const css = readProse();
+      assert.match(css, /\.prose\s+svg[^{]*\{[^}]*border-radius/s,
+        'SVGs should have border-radius');
+    });
+  });
+
+  describe('BaseLayout main element', () => {
+    it('should not constrain main to --width-container', () => {
+      const content = readLayout();
+      // The main style block should NOT have max-width: var(--width-container)
+      const mainBlock = content.match(/main\s*\{[^}]*\}/s);
+      assert.ok(mainBlock, 'main style block should exist');
+      assert.ok(
+        !mainBlock[0].includes('max-width'),
+        'main should not have max-width — child elements handle their own widths'
+      );
+    });
+  });
+
+  describe('Post template prose-wrapper', () => {
+    it('should not constrain prose-wrapper to --width-prose', () => {
+      const content = readPostTemplate();
+      const styleBlock = content.match(/<style>([\s\S]*?)<\/style>/);
+      assert.ok(styleBlock, 'style block should exist');
+      assert.ok(
+        !styleBlock[1].includes('max-width: var(--width-prose)'),
+        '.prose-wrapper should not be constrained to --width-prose (children self-constrain)'
+      );
+    });
+  });
+});
\ No newline at end of file