diff --git a/assets/css/extended/custom.css b/assets/css/extended/custom.css
index 2d3bbca..c105fc0 100644
--- a/assets/css/extended/custom.css
+++ b/assets/css/extended/custom.css
@@ -37,6 +37,46 @@ body {
     font-size: .7em;
 }
 
+
+.toc {
+    padding: 14px;
+    border: solid 1px lightgray;
+    font-size: 12px;
+}
+
+
+@media (min-width: 1280px) {
+    .toc {
+        position: sticky;
+        float: left;
+        --toc-left: calc(100vw / 50);
+        left: var(--toc-left); /* _minimum_ distance from left screen border */
+        top: 100px;
+        margin-left: -1000px; /* overruled by left */
+    
+        width: calc((100vw - var(--main-width) - 2 * var(--gap)) / 2 - 2 * var(--toc-left));
+        padding: 14px;
+        border: solid 1px lightgray;
+        font-size: 12px;
+    }
+  
+    .toc .inner {
+        padding: 0;
+    }
+  
+    .toc details summary {
+        margin-inline-start: 0;
+        margin-bottom: 10px;
+    }
+
+}
+
+
+
+summary {
+    cursor: pointer !important;
+}
+
 :root {
     --gap: 24px;
     --content-gap: 20px;
diff --git a/layouts/partials/toc.html b/layouts/partials/toc.html
new file mode 100644
index 0000000..4d6748d
--- /dev/null
+++ b/layouts/partials/toc.html
@@ -0,0 +1,93 @@
+  <div class="toc">
+    <details {{if (.Param "TocOpen") }} open{{ end }}>
+      <summary accesskey="c" title="(Alt + C)">
+        <div class="details">{{ .Title }}</div>
+      </summary>
+      <div class="inner">
+{{- $headers := findRE "<h[1-6].*?>(.|\n])+?</h[1-6]>" .Content -}}
+{{- $has_headers := ge (len $headers) 1 -}}
+{{- if $has_headers -}}
+
+{{- $largest := 6 -}}
+{{- range $headers -}}
+{{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
+{{- $headerLevel := len (seq $headerLevel) -}}
+{{- if lt $headerLevel $largest -}}
+{{- $largest = $headerLevel -}}
+{{- end -}}
+{{- end -}}
+
+{{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}}
+
+{{- $.Scratch.Set "bareul" slice -}}
+<ul>
+    {{- range seq (sub $firstHeaderLevel $largest) -}}
+    <ul>
+        {{- $.Scratch.Add "bareul" (sub (add $largest .) 1) -}}
+        {{- end -}}
+        {{- range $i, $header := $headers -}}
+        {{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
+        {{- $headerLevel := len (seq $headerLevel) -}}
+
+        {{/* get id="xyz" */}}
+        {{- $id := index (findRE "(id=\"(.*?)\")" $header 9) 0 }}
+
+        {{- /* strip id="" to leave xyz, no way to get regex capturing groups in hugo */ -}}
+        {{- $cleanedID := replace (replace $id "id=\"" "") "\"" "" }}
+        {{- $header := replaceRE "<h[1-6].*?>((.|\n])+?)</h[1-6]>" "$1" $header -}}
+
+        {{- if ne $i 0 -}}
+        {{- $prevHeaderLevel := index (findRE "[1-6]" (index $headers (sub $i 1)) 1) 0 -}}
+        {{- $prevHeaderLevel := len (seq $prevHeaderLevel) -}}
+        {{- if gt $headerLevel $prevHeaderLevel -}}
+        {{- range seq $prevHeaderLevel (sub $headerLevel 1) -}}
+        <ul>
+            {{/* the first should not be recorded */}}
+            {{- if ne $prevHeaderLevel . -}}
+            {{- $.Scratch.Add "bareul" . -}}
+            {{- end -}}
+            {{- end -}}
+            {{- else -}}
+            </li>
+            {{- if lt $headerLevel $prevHeaderLevel -}}
+            {{- range seq (sub $prevHeaderLevel 1) -1 $headerLevel -}}
+            {{- if in ($.Scratch.Get "bareul") . -}}
+        </ul>
+        {{/* manually do pop item */}}
+        {{- $tmp := $.Scratch.Get "bareul" -}}
+        {{- $.Scratch.Delete "bareul" -}}
+        {{- $.Scratch.Set "bareul" slice}}
+        {{- range seq (sub (len $tmp) 1) -}}
+        {{- $.Scratch.Add "bareul" (index $tmp (sub . 1)) -}}
+        {{- end -}}
+        {{- else -}}
+    </ul>
+    </li>
+    {{- end -}}
+    {{- end -}}
+    {{- end -}}
+    {{- end -}}
+    <li>
+        <a href="#{{- $cleanedID  -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
+        {{- else -}}
+    <li>
+        <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
+        {{- end -}}
+        {{- end -}}
+        <!-- {{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}} -->
+        {{- $firstHeaderLevel := $largest }}
+        {{- $lastHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers (sub (len $headers) 1)) 1) 0)) -}}
+    </li>
+    {{- range seq (sub $lastHeaderLevel $firstHeaderLevel) -}}
+    {{- if in ($.Scratch.Get "bareul") (add . $firstHeaderLevel) -}}
+</ul>
+{{- else -}}
+</ul>
+</li>
+{{- end -}}
+{{- end -}}
+</ul>
+{{- end -}}
+      </div>
+    </details>
+  </div>
\ No newline at end of file