diff --git a/public/buzzer.html b/public/buzzer.html
index 43afeeb..370941e 100644
--- a/public/buzzer.html
+++ b/public/buzzer.html
@@ -59,6 +59,10 @@
         ></textarea>
       </label>
     </div>
+    <div>
+      <label
+        >double:
+        <button data-onclick="conn.send('control', {action: 'double'})">double</button>
       </label>
     </div>
   </div>
diff --git a/public/matrix.js b/public/matrix.js
index b4a3e90..91e7099 100644
--- a/public/matrix.js
+++ b/public/matrix.js
@@ -1,5 +1,7 @@
 "use strict"
 
+import { qs } from "./shared.js"
+
 function setup() {
   let highlighted
   document.addEventListener("click", (event) => {
@@ -18,6 +20,18 @@ function setup() {
     highlighted = card
     card.classList.add("highlight")
   })
+
+  window.addEventListener("message", ({ data }) => {
+    if (data !== "double") {
+      return
+    }
+    for (let card of qs(".card")) {
+      if (card.classList.contains("cat")) {
+        continue
+      }
+      card.textContent = parseInt(card.textContent) * 2
+    }
+  })
 }
 
 setup()
diff --git a/public/monitor.html b/public/monitor.html
index 0c10494..d526c78 100644
--- a/public/monitor.html
+++ b/public/monitor.html
@@ -6,7 +6,7 @@
     <div id="players">
       <!-- players (see template#player) will be inserted here -->
     </div>
-    <iframe src="matrix.html"></iframe>
+    <iframe id="board" src="matrix.html"></iframe>
     <div id="text-overlay" class="hidden"></div>
   </div>
 </body>
diff --git a/public/monitor.js b/public/monitor.js
index 12db086..186ef39 100644
--- a/public/monitor.js
+++ b/public/monitor.js
@@ -174,6 +174,8 @@ function setup_ws() {
       } else if (action === "sound" && args.sound) {
         const { client_id, sound } = args
         sounds[client_id] = new Audio(`sounds/${sound}.mp3`)
+      } else if (action === "double") {
+        q("#board").contentWindow.postMessage("double", "*")
       }
     },
   )
diff --git a/public/shared.js b/public/shared.js
index 882dcc5..cc9cbd7 100644
--- a/public/shared.js
+++ b/public/shared.js
@@ -15,6 +15,7 @@ export function isEmpty(x) {
 }
 
 export const q = (selector, root = document) => root.querySelector(selector)
+export const qs = (selector, root = document) => root.querySelectorAll(selector)
 
 const _evlisteners = {}