diff --git a/.claude/settings.json b/.claude/settings.json
index 161080f..0bbf442 100644
--- a/.claude/settings.json
+++ b/.claude/settings.json
@@ -10,7 +10,8 @@
       "Bash(/Users/mattsp/projects/sound/build-web.sh)",
       "Bash(wasm-pack build:*)",
       "Read(//Users/mattsp/projects/sound/**)",
-      "Bash(./build-web.sh)"
+      "Bash(./build-web.sh)",
+      "Bash(python3 -c \"import sys,json; [print\\(json.loads\\(l\\).keys\\(\\)\\) for l in sys.stdin]\")"
     ]
   }
 }
diff --git a/crates/synth-core/src/audio_out.rs b/crates/synth-core/src/audio_out.rs
index 8197900..fc07528 100644
--- a/crates/synth-core/src/audio_out.rs
+++ b/crates/synth-core/src/audio_out.rs
@@ -25,6 +25,16 @@ impl AudioOut {
         params: &[
             ParamDescriptor { id: "level", label: "Level", min: 0.0, max: 1.0, default: 0.8, unit: "", labels: &[] },
         ],
+        description: "\
+## Output — Audio Sink
+
+The final destination in the signal chain, representing the speakers or DAC.
+
+Connect the last audio module's output jack to **In**. The green **● LIVE** badge lights up when a cable is connected and audio is flowing.
+
+**Level** controls the master output volume.
+
+> The oscilloscope and spectrum analyser always reflect what arrives at this node.",
     };
 }
 
diff --git a/crates/synth-core/src/descriptor.rs b/crates/synth-core/src/descriptor.rs
index 5eef161..ba4ab0b 100644
--- a/crates/synth-core/src/descriptor.rs
+++ b/crates/synth-core/src/descriptor.rs
@@ -71,4 +71,6 @@ pub struct ComponentDescriptor {
     pub jacks: &'static [JackDescriptor],
     /// Ordered list of tunable parameters.
     pub params: &'static [ParamDescriptor],
+    /// Markdown-formatted help text shown in the patch-bay tooltip.
+    pub description: &'static str,
 }
diff --git a/crates/synth-core/src/envelope.rs b/crates/synth-core/src/envelope.rs
index 0019d66..91c7f8a 100644
--- a/crates/synth-core/src/envelope.rs
+++ b/crates/synth-core/src/envelope.rs
@@ -39,6 +39,19 @@ impl Adsr {
             ParamDescriptor { id: "sustain",   label: "Sustain", min: 0.0,   max: 1.0, default: 0.7,  unit: "",  labels: &[] },
             ParamDescriptor { id: "release_s", label: "Release", min: 0.001, max: 8.0, default: 0.3,  unit: "s", labels: &[] },
         ],
+        description: "\
+## ADSR — Envelope Generator
+
+Produces a CV signal (0–1) that shapes amplitude over time in four stages:
+
+- **Attack** — time to rise from 0 to peak after gate opens
+- **Decay** — time to fall from peak to sustain level
+- **Sustain** — level held while gate remains open
+- **Release** — time to fall back to 0 after gate closes
+
+**This module outputs CV, not audio.** Connect **Env** to a VCA `cv_gain` jack to shape volume, or to `cv_cutoff_hz` on the Filter for a filter envelope.
+
+> **Tip:** VCO Out → VCA In, Keyboard Gate → ADSR Gate, ADSR Env → VCA cv_gain, VCA Out → Output In",
     };
 
     pub fn gate_on(&mut self)  { self.stage = Stage::Attack; }
diff --git a/crates/synth-core/src/filter.rs b/crates/synth-core/src/filter.rs
index 67362ed..6aceb97 100644
--- a/crates/synth-core/src/filter.rs
+++ b/crates/synth-core/src/filter.rs
@@ -42,6 +42,16 @@ impl Svf {
             ParamDescriptor { id: "cutoff_hz", label: "Cutoff", min: 20.0, max: 20_000.0, default: 2_000.0, unit: "Hz", labels: &[] },
             ParamDescriptor { id: "resonance", label: "Res",    min: 0.0,  max: 1.0,      default: 0.5,     unit: "",   labels: &[] },
         ],
+        description: "\
+## Filter (SVF) — State-Variable Filter
+
+2-pole filter that shapes the tonal colour of an audio signal.
+
+**In** accepts audio; **Out** emits the filtered result.
+
+**Cutoff** sets the corner frequency — frequencies above this point are attenuated (in low-pass mode). **Res** (resonance) narrows the peak; at 1.0 the filter self-oscillates.
+
+Connect an LFO to `cv_cutoff_hz` for a sweeping wah effect, or an ADSR for a filter envelope.",
     };
 
     #[inline]
diff --git a/crates/synth-core/src/lfo.rs b/crates/synth-core/src/lfo.rs
index 5e0e366..ff7b09b 100644
--- a/crates/synth-core/src/lfo.rs
+++ b/crates/synth-core/src/lfo.rs
@@ -31,6 +31,18 @@ impl Lfo {
             ParamDescriptor { id: "depth",    label: "Depth", min: 0.0,  max: 1.0,  default: 0.5, unit: "",   labels: &[] },
             ParamDescriptor { id: "waveform", label: "Wave",  min: 0.0,  max: 4.0,  default: 0.0, unit: "",   labels: &["Sine", "Saw", "Sqr", "Tri", "Pls"] },
         ],
+        description: "\
+## LFO — Low-Frequency Oscillator
+
+Like a VCO but running at sub-audio speeds (0.01–20 Hz). Outputs a CV signal (−1 to +1) for modulation.
+
+**Rate** controls oscillation speed. **Depth** scales the output amplitude (0 = no modulation, 1 = full swing).
+
+**Typical uses:**
+
+- LFO Out → VCO `cv_freq_hz` for **vibrato** (pitch wobble)
+- LFO Out → Filter `cv_cutoff_hz` for a **wah/sweep** effect
+- LFO Out → VCA `cv_gain` for **tremolo** (volume wobble)",
     };
 
     #[inline]
diff --git a/crates/synth-core/src/oscillator.rs b/crates/synth-core/src/oscillator.rs
index 1e1c4a1..af1ce2b 100644
--- a/crates/synth-core/src/oscillator.rs
+++ b/crates/synth-core/src/oscillator.rs
@@ -37,6 +37,16 @@ impl Vco {
             ParamDescriptor { id: "freq_hz",  label: "Freq", min: 20.0, max: 20_000.0, default: 440.0, unit: "Hz", labels: &[] },
             ParamDescriptor { id: "waveform", label: "Wave", min: 0.0,  max: 4.0,      default: 1.0,   unit: "",   labels: &["Sine", "Saw", "Sqr", "Tri", "Pls"] },
         ],
+        description: "\
+## VCO — Voltage-Controlled Oscillator
+
+Generates a periodic audio-rate waveform at the set frequency.
+
+**Waveforms:** Sine · Saw · Square · Triangle · Pulse
+
+**Freq** sets the base pitch. Connect an LFO or keyboard CV to `cv_freq_hz` for 1 V/oct pitch modulation (0 V = 440 Hz, +1 V = 880 Hz).
+
+**Typical chain:** VCO Out → Filter In → VCA In → Output In",
     };
 
     #[inline]
diff --git a/crates/synth-core/src/vca.rs b/crates/synth-core/src/vca.rs
index f9c6a1a..63df4b7 100644
--- a/crates/synth-core/src/vca.rs
+++ b/crates/synth-core/src/vca.rs
@@ -22,6 +22,16 @@ impl Vca {
         params: &[
             ParamDescriptor { id: "gain", label: "Gain", min: 0.0, max: 1.0, default: 1.0, unit: "", labels: &[] },
         ],
+        description: "\
+## VCA — Voltage-Controlled Amplifier
+
+Scales the audio level of a signal. Audio enters on **In** and exits at a controlled volume on **Out**.
+
+**Gain** knob sets the base level (0 = silence, 1 = unity gain).
+
+Connect an ADSR **Env** output to the `cv_gain` jack to apply an amplitude envelope — the CV value multiplies with the knob gain each sample.
+
+**Typical chain:** VCO Out → VCA In → Output In, with ADSR Env → VCA cv_gain",
     };
 }
 
diff --git a/crates/synth-visualiser/src/patchbay.rs b/crates/synth-visualiser/src/patchbay.rs
index d54a2ca..cc272ba 100644
--- a/crates/synth-visualiser/src/patchbay.rs
+++ b/crates/synth-visualiser/src/patchbay.rs
@@ -141,6 +141,13 @@ impl Module {
             && y >= self.y && y <= self.y + MOD_HEADER
     }
 
+    /// Hit-test the `?` help badge drawn in the top-right of the header.
+    fn hit_help_badge(&self, x: f32, y: f32) -> bool {
+        let bx = self.x + MOD_W - 10.0;
+        let by = self.y + MOD_HEADER / 2.0;
+        hypot(x - bx, y - by) <= 9.0
+    }
+
     fn hit_body(&self, x: f32, y: f32) -> bool {
         x >= self.x && x <= self.x + MOD_W
             && y >= self.y && y <= self.y + self.height()
@@ -490,6 +497,18 @@ impl PatchBay {
         }
     }
 
+    /// Returns the markdown description for the module whose `?` badge is at
+    /// (x, y), or an empty string if none is hovered.  JS calls this on every
+    /// `mousemove` over the patch bay canvas to drive the tooltip overlay.
+    pub fn get_tooltip_at(&self, x: f32, y: f32) -> String {
+        for m in self.modules.iter().rev() {
+            if m.hit_help_badge(x, y) {
+                return m.descriptor().description.to_string();
+            }
+        }
+        String::new()
+    }
+
     /// Double-click removes the topmost module under the cursor.
     pub fn on_double_click(&mut self, x: f32, y: f32) {
         if y < PALETTE_H { return; }
@@ -685,7 +704,25 @@ impl PatchBay {
             (m.y + MOD_HEADER * 0.70) as f64,
         );
 
+        // ? help badge — top-right corner of every module header
+        {
+            let bx = (m.x + MOD_W - 10.0) as f64;
+            let by = (m.y + MOD_HEADER / 2.0) as f64;
+            ctx.begin_path();
+            let _ = ctx.arc(bx, by, 7.0, 0.0, core::f64::consts::TAU);
+            ctx.set_fill_style_str("rgba(0,0,0,0.30)");
+            ctx.fill();
+            ctx.set_stroke_style_str("rgba(255,255,255,0.25)");
+            ctx.set_line_width(1.0);
+            ctx.stroke();
+            ctx.set_fill_style_str("rgba(255,255,255,0.70)");
+            ctx.set_font("bold 9px sans-serif");
+            ctx.set_text_align("center");
+            let _ = ctx.fill_text("?", bx, by + 3.5);
+        }
+
         // Output node: LIVE / UNPATCHED badge on the right of the header
+        // (shifted left to clear the ? badge)
         if is_out {
             if has_signal {
                 ctx.set_fill_style_str("#22c55e");
@@ -693,7 +730,7 @@ impl PatchBay {
                 ctx.set_text_align("right");
                 let _ = ctx.fill_text(
                     "● LIVE",
-                    (m.x + MOD_W - 6.0) as f64,
+                    (m.x + MOD_W - 24.0) as f64,
                     (m.y + MOD_HEADER * 0.70) as f64,
                 );
             } else {
@@ -702,7 +739,7 @@ impl PatchBay {
                 ctx.set_text_align("right");
                 let _ = ctx.fill_text(
                     "unpatched",
-                    (m.x + MOD_W - 6.0) as f64,
+                    (m.x + MOD_W - 24.0) as f64,
                     (m.y + MOD_HEADER * 0.70) as f64,
                 );
             }
diff --git a/www/bootstrap.js b/www/bootstrap.js
index 64037c5..9711064 100644
--- a/www/bootstrap.js
+++ b/www/bootstrap.js
@@ -28,6 +28,86 @@ function fitCanvas(canvas) {
     }
 }
 
+// ── Tooltip helpers ───────────────────────────────────────────────────────────
+
+/** Minimal markdown → HTML converter for the module description tooltips. */
+function mdToHtml(md) {
+    const lines = md.split("\n");
+    const out   = [];
+    let inUl    = false;
+
+    for (const raw of lines) {
+        const line = raw.trimEnd();
+
+        if (line.startsWith("## ")) {
+            if (inUl) { out.push("</ul>"); inUl = false; }
+            out.push(`<h2>${esc(line.slice(3))}</h2>`);
+        } else if (line.startsWith("> ")) {
+            if (inUl) { out.push("</ul>"); inUl = false; }
+            out.push(`<blockquote>${inline(line.slice(2))}</blockquote>`);
+        } else if (line.startsWith("- ")) {
+            if (!inUl) { out.push("<ul>"); inUl = true; }
+            out.push(`<li>${inline(line.slice(2))}</li>`);
+        } else {
+            if (inUl) { out.push("</ul>"); inUl = false; }
+            if (line === "") {
+                out.push("<br>");
+            } else {
+                out.push(`<p>${inline(line)}</p>`);
+            }
+        }
+    }
+    if (inUl) out.push("</ul>");
+    return out.join("");
+}
+
+function esc(s) {
+    return s.replace(/&/g,"&amp;").replace(/</g,"&lt;").replace(/>/g,"&gt;");
+}
+
+function inline(s) {
+    // `code`, **bold**
+    return esc(s)
+        .replace(/`([^`]+)`/g, "<code>$1</code>")
+        .replace(/\*\*([^*]+)\*\*/g, "<strong>$1</strong>");
+}
+
+const _tooltip = { el: null };
+
+function getTooltipEl() {
+    if (!_tooltip.el) _tooltip.el = document.getElementById("tooltip");
+    return _tooltip.el;
+}
+
+function updateTooltip(clientX, clientY, markdown) {
+    const el = getTooltipEl();
+    if (!el) return;
+    if (!markdown) { hideTooltip(); return; }
+
+    el.innerHTML = mdToHtml(markdown);
+    el.style.display = "block";
+
+    // Position: prefer right of cursor, flip left if off-screen
+    const margin = 14;
+    const vw = window.innerWidth;
+    const vh = window.innerHeight;
+    let left = clientX + margin;
+    let top  = clientY + margin;
+    el.style.left = "0";
+    el.style.top  = "0";
+    const tw = el.offsetWidth;
+    const th = el.offsetHeight;
+    if (left + tw > vw - 4) left = clientX - tw - margin;
+    if (top  + th > vh - 4) top  = clientY - th - margin;
+    el.style.left = left + "px";
+    el.style.top  = top  + "px";
+}
+
+function hideTooltip() {
+    const el = getTooltipEl();
+    if (el) el.style.display = "none";
+}
+
 // ── Resize handle ─────────────────────────────────────────────────────────────
 
 function initResizeHandle() {
@@ -434,8 +514,12 @@ async function bootstrap() {
 
         // ── Patch bay pointer events ──────────────────────────────────────────
         pbCanvas.addEventListener("pointerdown",  e => patchbay.on_pointer_down(e.offsetX, e.offsetY));
-        pbCanvas.addEventListener("pointermove",  e => patchbay.on_pointer_move(e.offsetX, e.offsetY));
+        pbCanvas.addEventListener("pointermove",  e => {
+            patchbay.on_pointer_move(e.offsetX, e.offsetY);
+            updateTooltip(e.clientX, e.clientY, patchbay.get_tooltip_at(e.offsetX, e.offsetY));
+        });
         pbCanvas.addEventListener("pointerup",    e => patchbay.on_pointer_up(e.offsetX, e.offsetY));
+        pbCanvas.addEventListener("pointerleave", ()  => hideTooltip());
         pbCanvas.addEventListener("dblclick",     e => patchbay.on_double_click(e.offsetX, e.offsetY));
 
         // ── Patch router: Web Audio graph driven by patch bay topology ────────
diff --git a/www/index.html b/www/index.html
index 89b7e67..20eb99c 100644
--- a/www/index.html
+++ b/www/index.html
@@ -151,10 +151,50 @@
       z-index: 100; transition: opacity 0.4s;
     }
     #loader.hidden { opacity: 0; pointer-events: none; }
+
+    #tooltip {
+      position: fixed;
+      z-index: 200;
+      max-width: 320px;
+      background: #1a1e2c;
+      border: 1px solid #3b4560;
+      border-radius: 8px;
+      padding: 12px 14px;
+      font-size: 0.72rem;
+      line-height: 1.65;
+      color: #c8d0e0;
+      pointer-events: none;
+      box-shadow: 0 6px 28px rgba(0,0,0,0.75);
+      display: none;
+    }
+    #tooltip h2 {
+      font-size: 0.78rem;
+      color: #e8eaf6;
+      margin-bottom: 6px;
+      letter-spacing: 0.04em;
+    }
+    #tooltip strong { color: #a5d8ff; }
+    #tooltip code {
+      background: rgba(255,255,255,0.08);
+      border-radius: 3px;
+      padding: 0 3px;
+      font-size: 0.68rem;
+      color: #94e2d5;
+    }
+    #tooltip blockquote {
+      border-left: 2px solid #3b4560;
+      margin: 6px 0 0;
+      padding-left: 8px;
+      color: #8899aa;
+    }
+    #tooltip ul {
+      margin: 4px 0 0 14px;
+    }
   </style>
 </head>
 <body>
   <div id="loader">Loading WASM module…</div>
+  <div id="tooltip"></div>
 
   <header>Analogue Synth Visualiser</header>