diff --git a/web/package.json b/web/package.json
index 5e2d67d..717c339 100644
--- a/web/package.json
+++ b/web/package.json
@@ -7,7 +7,7 @@
     "dev": "vite",
     "build": "tsc --noEmit && vite build",
     "icons": "node scripts/icons.mjs",
-    "test": "node scripts/test-graphcols.mjs && node scripts/test-css-imports.mjs && node scripts/test-resolve-path.mjs"
+    "test": "node scripts/test-graphcols.mjs && node scripts/test-sockets.mjs && node scripts/test-css-imports.mjs && node scripts/test-resolve-path.mjs"
   },
   "dependencies": {
     "@xterm/addon-fit": "^0.10.0",
diff --git a/web/scripts/test-sockets.mjs b/web/scripts/test-sockets.mjs
new file mode 100644
index 0000000..c8b5d10
--- /dev/null
+++ b/web/scripts/test-sockets.mjs
@@ -0,0 +1,34 @@
+// node scripts/test-sockets.mjs — src/sockets.ts: each CPU's threads, counted under it.
+import { buildSync } from 'esbuild'
+import { pathToFileURL } from 'node:url'
+import { mkdtempSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+
+const out = join(mkdtempSync(join(tmpdir(), 'sockets-')), 'sockets.mjs')
+buildSync({ entryPoints: [new URL('../src/sockets.ts', import.meta.url).pathname], outfile: out, format: 'esm', logLevel: 'error' })
+const { socketThreads } = await import(pathToFileURL(out).href)
+
+const fails = []
+const eq = (got, want, what) => { if (JSON.stringify(got) !== JSON.stringify(want)) fails.push(`${what} — got ${JSON.stringify(got)}, want ${JSON.stringify(want)}`) }
+const range = (a, b) => Array.from({ length: b - a }, (_, i) => a + i)
+
+// nexus: 2 × E5-2680 v2, as /sys/devices/system/cpu/cpu*/topology/physical_package_id reads
+const nexus = [...Array(10).fill(0), ...Array(10).fill(1), ...Array(10).fill(0), ...Array(10).fill(1)]
+const got = socketThreads(nexus)
+eq(got.map((s) => s.pkg), [0, 1], 'two sockets, in order')
+eq(got[0].cpus, [...range(0, 10), ...range(20, 30)], "CPU 1's threads")
+eq(got[1].cpus, [...range(10, 20), ...range(30, 40)], "CPU 2's threads")
+eq(got.map((s) => s.cpus.length), [20, 20], 'twenty threads each')
+// every thread is under exactly one CPU
+eq(got.flatMap((s) => s.cpus).sort((a, b) => a - b), range(0, 40), 'every thread once')
+// a thread's number under its CPU runs 0…19 on both
+for (const s of got) eq(s.cpus.map((_, n) => n), range(0, 20), `CPU ${s.pkg + 1} counts from 0`)
+
+// a board that numbers each socket's threads together, and one whose socket ids are not 0 and 1
+eq(socketThreads([0, 0, 0, 0, 1, 1, 1, 1]), [{ pkg: 0, cpus: [0, 1, 2, 3] }, { pkg: 1, cpus: [4, 5, 6, 7] }], 'contiguous numbering')
+eq(socketThreads([3, 1, 3, 1]), [{ pkg: 1, cpus: [1, 3] }, { pkg: 3, cpus: [0, 2] }], 'socket ids sorted')
+eq(socketThreads([]), [], 'nothing')
+
+if (fails.length) { console.error('sockets: FAIL\n  ' + fails.join('\n  ')); process.exit(1) }
+console.log('sockets: ok')
diff --git a/web/src/Top.tsx b/web/src/Top.tsx
index eaecf5a..a947b00 100644
--- a/web/src/Top.tsx
+++ b/web/src/Top.tsx
@@ -11,6 +11,7 @@ import { fmtMs, nextPoint, REFRESH_OPTIONS, setRefreshMs, strideMs, useHistorySe
 import { Loading, askYes, toast } from './ui'
 import { Theme, currentThemeId, themeById } from './themes'
 import { columnsAt } from './graphcols'
+import { socketThreads } from './sockets'
 import { savePrefs } from './accents'
 
 const MAX = 300 // points kept per curve, as the machines keep them
@@ -370,7 +371,10 @@ export default function Top() {
   // On a two-socket board each CPU has its own sensor chip, both counting from "Core 0", so a
   // thread's core is looked up on its own socket's chip.
   const pkgs = pt?.core_pkgs?.length === cores.length ? pt.core_pkgs : undefined
-  const sockets = pkgs ? [...new Set(pkgs)].sort((a, b) => a - b) : []
+  // each CPU's own threads, counted from 0 under it (sockets.ts): the kernel's one numbering
+  // of the whole machine puts cpu 0–9 and 20–29 on the first and 10–19 and 30–39 on the second
+  const socks = pkgs ? socketThreads(pkgs) : []
+  const sockets = socks.map((x) => x.pkg)
   const coreTemp = (i: number) => { const id = pt?.core_ids?.[i] ?? i, pk = pkgs?.[i]; return s?.temps.find((t) => new RegExp(`core ${id}\\b`, 'i').test(t.label) && (pk === undefined || (t.pkg ?? 0) === pk))?.c }
   const sockTemp = (pk: number) => s?.temps.find((t) => t.chip === 'coretemp' && new RegExp(`^package id ${pk}$`, 'i').test(t.label))?.c
   const sockUse = (pk: number) => { const v = cores.filter((_, i) => pkgs?.[i] === pk); return v.length ? v.reduce((a, b) => a + b, 0) / v.length : 0 }
@@ -425,7 +429,7 @@ export default function Top() {
               <span className="t-core-p">{Math.round(cpu)}%</span>
               {anyTemp && <><span className="t-core-t">{cpuTemp !== undefined && <Meter pct={Math.min(100, (cpuTemp / 100) * 100)} grad="temp" />}</span><span className="t-core-c">{cpuTemp !== undefined ? `${Math.round(cpuTemp)}°C` : ''}</span></>}
             </div>}
-            {(sockets.length > 1 ? sockets : [undefined]).map((pk) => (
+            {(sockets.length > 1 ? socks : [{ pkg: undefined, cpus: cores.map((_, i) => i) }]).map(({ pkg: pk, cpus }) => (
               <Fragment key={pk ?? 'all'}>
                 {pk !== undefined && (() => {
                   const u = sockUse(pk), t = sockTemp(pk), w = pt?.cpu_power_pkgs_w?.[pk], open = !folded.includes(pk)
@@ -439,13 +443,13 @@ export default function Top() {
                     </div>
                   )
                 })()}
-                {cores.map((v, i) => {
-                  if (pk !== undefined && (pkgs?.[i] !== pk || folded.includes(pk))) return null
-                  const t = coreTemp(i)
+                {(pk !== undefined && folded.includes(pk) ? [] : cpus).map((i, n) => {
+                  // n is the thread's number under its own CPU, i the kernel's for the whole machine
+                  const t = coreTemp(i), v = cores[i] ?? 0
                   return (
                     <div className="t-core" key={i}>
-                      <span className="t-core-n">C{i}</span>
-                      <span className="t-core-g" title={(pk !== undefined ? `CPU ${pk + 1} · ` : '') + (pt?.core_ids && pt.core_ids[i] !== i ? `cpu ${i} · a thread of core ${pt.core_ids[i]}` : `core ${i}`)}>{braille(recent.get(`cpu.core.${i}.usage`) ?? [], 9)}</span>
+                      <span className="t-core-n">C{n}</span>
+                      <span className="t-core-g" title={pk !== undefined ? `CPU ${pk + 1} · thread ${n} · cpu ${i} to the kernel${pt?.core_ids ? ` · a thread of core ${pt.core_ids[i]}` : ''}` : pt?.core_ids && pt.core_ids[i] !== i ? `cpu ${i} · a thread of core ${pt.core_ids[i]}` : `core ${i}`}>{braille(recent.get(`cpu.core.${i}.usage`) ?? [], 9)}</span>
                       <span className="t-core-p">{Math.round(v)}%</span>
                       {anyTemp && <><span className="t-core-t">{t !== undefined && <Meter pct={Math.min(100, t)} grad="temp" />}</span><span className="t-core-c">{t !== undefined ? `${Math.round(t)}°C` : ''}</span></>}
                     </div>
diff --git a/web/src/sockets.ts b/web/src/sockets.ts
new file mode 100644
index 0000000..df5eaf2
--- /dev/null
+++ b/web/src/sockets.ts
@@ -0,0 +1,17 @@
+// Which threads belong to which CPU, on a machine with more than one.
+//
+// The kernel numbers every thread of the machine in one run, and on a two-socket board with
+// hyper-threading it deals them out a socket at a time and then goes round again: cpu 0–9 are
+// CPU 1's cores, 10–19 CPU 2's, 20–29 the second threads of CPU 1's, 30–39 of CPU 2's. Listed
+// under its CPU by that number, a 20-thread processor reads "C0…C9, C20…C29" and the one
+// beside it "C10…C19, C30…C39" — neither looks like twenty of anything. So each CPU's threads
+// are counted from 0 under it, in the kernel's order, the way a one-socket machine's are: the
+// first half are its cores, the second half their second threads.
+//
+// `pkgs[i]` is the socket logical CPU `i` is on. The answer is, for each socket (ascending),
+// the logical CPUs on it (ascending); a thread's number under its CPU is its place in that list.
+export function socketThreads(pkgs: number[]): { pkg: number; cpus: number[] }[] {
+  const by = new Map<number, number[]>()
+  pkgs.forEach((pk, i) => { const l = by.get(pk); if (l) l.push(i); else by.set(pk, [i]) })
+  return [...by.entries()].sort((a, b) => a[0] - b[0]).map(([pkg, cpus]) => ({ pkg, cpus }))
+}