diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 936d024..994ca97 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -30,6 +30,7 @@ Contributions from everyone are welcomed. To keep the roadmap practical, accessi
 - Contributions should match the topics and structure of the roadmap.
 - Areas where contributors have experience or genuine interest should be focused on.
 - If a new topic is thought to make the roadmap more complete, it may be suggested. New topics should be proposed thoughtfully, considering their usefulness and relevance for other learners.
+- Connections between topics on the map are listed in `site/map.json` under `links` as `["Topic", "Other topic", "why they are related"]`, using the names shown on the map. Connect topics that a learner should study together or that depend on each other, and keep the reason to one short sentence. Tests fail if a name is misspelled, a pair is listed twice or a topic has no connection.
 
 ## 6. Versioning and Releases
 
diff --git a/site/app.js b/site/app.js
index 149a017..4bd2a51 100644
--- a/site/app.js
+++ b/site/app.js
@@ -155,8 +155,33 @@ function topicChips(nodes) {
   }, swatch(n), n.title, h('span', { class: 'n' }, n.total))));
 }
 
-function related(node) {
+// Hand-picked connections of a topic (from map.json), each with the reason it is listed.
+function relatedTopics(node) {
+  const seen = new Set();
+  const items = [];
+  for (const box of node.boxes || []) {
+    for (const { box: other, reason } of box.related || []) {
+      if (seen.has(other)) continue;
+      seen.add(other);
+      items.push({ other, reason });
+    }
+  }
+  return items;
+}
+
+function curatedList(items) {
+  return h('ul', { class: 'related' }, items.map(({ other, reason }) => {
+    const target = other.topics[0];
+    return h('li', {}, h('button', { type: 'button', onclick: () => navigate(target, { reveal: true }) },
+      swatch(target),
+      h('span', {}, other.text),
+      h('span', { class: 'why' }, reason)));
+  }));
+}
+
+function related(node, skip = new Set()) {
   const items = [...node.links]
+    .filter((link) => !skip.has(link.source === node ? link.target : link.source))
     .sort((a, b) => b.weight - a.weight)
     .map((link) => {
       const other = link.source === node ? link.target : link.source;
@@ -282,13 +307,13 @@ function overview(root) {
     section(null, h('div', { class: 'stats' },
       h('div', { class: 'stat' }, h('b', {}, stats.topics), h('span', {}, 'topics')),
       h('div', { class: 'stat' }, h('b', {}, stats.resources), h('span', {}, 'resources')),
-      h('div', { class: 'stat' }, h('b', {}, stats.links), h('span', {}, 'connections')))),
+      h('div', { class: 'stat' }, h('b', {}, state.plan ? state.plan.connections.filter((c) => c.reason).length : stats.links), h('span', {}, 'connections')))),
     state.plan ? section('Diagram topics by area', areaSummary()) : null,
     ...groups.map((g) => section(g.title, topicChips(g.topics))),
     section('How to explore', h('ul', { class: 'howto' },
       h('li', {}, 'Map follows the roadmap diagram: topics sit in Software, Hardware or both, and are colored by importance.'),
       h('li', {}, 'Use the Areas chips to highlight a single area or only the Software ∩ Hardware cross-section.'),
-      h('li', {}, 'Hover or select a topic to see lines to the topics it shares resources with (solid) or is mentioned by (dotted).'),
+      h('li', {}, 'Hover or select a topic to see solid lines to its related topics. Fainter dashed lines come from resources that share or mention a topic.'),
       h('li', {}, 'Outline lists every topic of the README; use it to browse topics that are not on the map.'),
       h('li', {}, 'Press ', h('kbd', {}, '/'), ' to search topics and resources.'))),
     section(null, h('p', { class: 'res-note' }, 'Built from the ',
@@ -325,7 +350,7 @@ function renderPanel(node) {
       node.importance ? h('span', { class: `pill imp-pill ${node.importance}` }, IMPORTANCE[node.importance]) : null,
       h('span', { class: 'pill' }, plural(node.total, 'resource')),
       node.children.length ? h('span', { class: 'pill' }, plural(node.children.length, 'subtopic')) : null,
-      node.links.length ? h('span', { class: 'pill' }, `${node.links.length} connected`) : null)));
+      relatedTopics(node).length ? h('span', { class: 'pill' }, `${relatedTopics(node).length} related`) : null)));
 
   if (node === root) {
     panel.append(...overview(root));
@@ -333,7 +358,11 @@ function renderPanel(node) {
   }
   if (node.description.length) panel.append(section('About', description(node.description)));
   if (node.children.length) panel.append(section('Subtopics', topicChips(node.children)));
-  if (node.links.length) panel.append(section('Connected topics', related(node)));
+  const curated = relatedTopics(node);
+  if (curated.length) panel.append(section('Related topics', curatedList(curated)));
+  const listed = new Set(curated.flatMap(({ other }) => other.topics));
+  const evidence = node.links.filter((link) => !listed.has(link.source === node ? link.target : link.source));
+  if (evidence.length) panel.append(section(curated.length ? 'Also linked by resources' : 'Linked by resources', related(node, listed)));
   panel.append(resources(node));
 }
 
diff --git a/site/diagram.js b/site/diagram.js
index 63980b8..365e315 100644
--- a/site/diagram.js
+++ b/site/diagram.js
@@ -118,15 +118,41 @@ export function linkDiagram(map, data) {
     return null;
   };
 
+  // A connection joins two boxes. `reason` is set for the hand-picked ones in map.json; the rest
+  // are evidence only (a shared resource, or one topic's text naming another).
   const pairs = new Map();
+  const pairFor = (a, b) => {
+    const [p, q] = a.id < b.id ? [a, b] : [b, a];
+    const id = `${p.id}|${q.id}`;
+    if (!pairs.has(id)) pairs.set(id, { a: p, b: q, reason: null, weight: 0, shared: 0, links: [] });
+    return pairs.get(id);
+  };
+
+  const topicBoxes = new Map(boxes.filter((b) => !b.header).map((b) => [b.text, b]));
+  const invalidLinks = [];
+  for (const box of boxes) box.related = [];
+  for (const [from, to, reason] of map.links || []) {
+    const a = topicBoxes.get(from);
+    const b = topicBoxes.get(to);
+    if (!a || !b || a === b || typeof reason !== 'string' || !reason.trim()) {
+      invalidLinks.push(`${from} - ${to}`);
+      continue;
+    }
+    const pair = pairFor(a, b);
+    if (pair.reason) {
+      invalidLinks.push(`${from} - ${to} (listed twice)`);
+      continue;
+    }
+    pair.reason = reason.trim();
+    a.related.push({ box: b, reason: pair.reason });
+    b.related.push({ box: a, reason: pair.reason });
+  }
+
   for (const link of data.links) {
     const a = boxFor(link.source);
     const b = boxFor(link.target);
     if (!a || !b || a === b) continue;
-    const [p, q] = a.id < b.id ? [a, b] : [b, a];
-    const id = `${p.id}|${q.id}`;
-    if (!pairs.has(id)) pairs.set(id, { a: p, b: q, weight: 0, shared: 0, links: [] });
-    const pair = pairs.get(id);
+    const pair = pairFor(a, b);
     pair.weight += link.weight;
     pair.shared += link.shared.length;
     pair.links.push(link);
@@ -139,6 +165,7 @@ export function linkDiagram(map, data) {
     boxFor,
     connections: [...pairs.values()],
     unmatched: boxes.filter((b) => !b.header && !b.topics.length).map((b) => b.text),
+    invalidLinks,
     invalid: boxes.filter((b) => !b.header && !IMPORTANCE_LEVELS.includes(b.importance)).map((b) => `${b.text}: ${b.importance}`),
   };
 }
diff --git a/site/map.css b/site/map.css
index b4c3901..7a252e6 100644
--- a/site/map.css
+++ b/site/map.css
@@ -325,15 +325,25 @@
   pointer-events: none;
 }
 
-.link.mention,
-.link-all.mention {
+.link.evidence,
+.link-all.evidence {
   stroke-dasharray: 5 5;
+  stroke-opacity: 0.45;
+}
+
+.link-end {
+  fill: var(--accent);
 }
 
 .link-all {
   stroke-opacity: 0.4;
 }
 
+.link-all.curated {
+  stroke-width: 1.2px;
+  stroke-opacity: 0.28;
+}
+
 .layer-linksAll {
   opacity: 0;
   transition: opacity 0.25s;
@@ -347,10 +357,6 @@
   opacity: 0.2;
 }
 
-.link-end {
-  fill: var(--accent);
-}
-
 .credit-main {
   fill: var(--text);
   font-size: 14px;
diff --git a/site/mapview.js b/site/mapview.js
index 5bdd21a..652ac6f 100644
--- a/site/mapview.js
+++ b/site/mapview.js
@@ -293,7 +293,7 @@ export function createMapView({ root, plan, data, build, onSelect }) {
       h('strong', {}, topic.title),
       topic.areas.length ? h('div', { class: 'tip-areas' }, areaDots(topic.areas), areaLabel(topic.areas)) : null,
       h('div', { class: 'tip-meta' }, [importance ? IMPORTANCE[importance] : null, plural(topic.total, 'resource'),
-        topic.links && topic.links.length ? `${topic.links.length} connected` : null].filter(Boolean).join(' · ')),
+        box && box.related.length ? `${box.related.length} related` : null].filter(Boolean).join(' · ')),
       h('div', { class: 'tip-hint' }, 'Click this topic to open its details'),
     ].filter(Boolean));
     tip.hidden = false;
@@ -325,8 +325,8 @@ export function createMapView({ root, plan, data, build, onSelect }) {
   function drawConnection(connection, layer, cls) {
     svg('path', {
       d: curve(connection.a, connection.b),
-      class: `${cls} ${connection.shared ? 'shared' : 'mention'}`,
-      'stroke-width': (1.4 + Math.min(3, Math.sqrt(connection.weight) * 0.7)).toFixed(2),
+      class: `${cls} ${connection.reason ? 'curated' : 'evidence'}`,
+      'stroke-width': connection.reason ? 2.4 : 1.4,
     }, layer);
     if (cls !== 'link') return;
     for (const box of [connection.a, connection.b]) {
@@ -335,7 +335,8 @@ export function createMapView({ root, plan, data, build, onSelect }) {
     }
   }
 
-  for (const connection of connections) drawConnection(connection, layers.linksAll, 'link-all');
+  // The overview of all connections only shows the hand-picked ones.
+  for (const connection of connections) if (connection.reason) drawConnection(connection, layers.linksAll, 'link-all');
 
   const isWithin = (node, topic) => {
     for (let n = node; n; n = n.parent) if (n === topic) return true;
@@ -353,7 +354,7 @@ export function createMapView({ root, plan, data, build, onSelect }) {
       for (const b of own) lit.add(b);
       for (const connection of connections) {
         if (!(own.has(connection.a) || own.has(connection.b))) continue;
-        if (!connection.links.some((l) => isWithin(l.source, topic) || isWithin(l.target, topic))) continue;
+        if (!connection.reason && !connection.links.some((l) => isWithin(l.source, topic) || isWithin(l.target, topic))) continue;
         drawConnection(connection, layers.links, 'link');
         lit.add(connection.a);
         lit.add(connection.b);
diff --git a/site/parser.test.mjs b/site/parser.test.mjs
index 0919ae4..c8426cf 100644
--- a/site/parser.test.mjs
+++ b/site/parser.test.mjs
@@ -71,6 +71,17 @@ test('places topics in the map areas and their cross-section', () => {
   assert.equal(find('Projects').areas.length, 0, 'getting-started topics are not on the map');
 });
 
+test('every map topic is connected to related topics, each connection with a reason', () => {
+  const plan = linkDiagram(map, data);
+  assert.deepEqual(plan.invalidLinks, [], 'map.json "links" name existing topics, once, with a reason');
+  // Soft skills describe the engineer, not the technology, so some of them stand alone.
+  const alone = plan.boxes.filter((b) => !b.header && !b.areas.includes('SOFT SKILLS') && !b.related.length);
+  assert.deepEqual(alone.map((b) => b.text), [], 'topics without a connection');
+  const has = (a, b) => plan.connections.some((c) => c.reason && [c.a.text, c.b.text].sort().join('|') === [a, b].sort().join('|'));
+  assert.ok(has('C', 'Memory Management'));
+  assert.ok(has('Rust', 'Memory Management'));
+});
+
 test('lays the map out on one A4 page with a minimum spacing between boxes', () => {
   const layout = buildLayout(linkDiagram(map, data));
   assert.deepEqual(layout.missing, [], 'every cluster in map.json has a place in layout.js');
diff --git a/site/styles.css b/site/styles.css
index 2ef0ec8..536f786 100644
--- a/site/styles.css
+++ b/site/styles.css
@@ -717,11 +717,9 @@ button {
 
 .related .why {
   grid-column: 2;
-  overflow: hidden;
   color: var(--muted);
   font-size: 12px;
-  text-overflow: ellipsis;
-  white-space: nowrap;
+  line-height: 1.4;
 }
 
 .res-head {