diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 8485160..21f6fbf 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -24,6 +24,7 @@ jobs:
         run: |
           node tests/base-converter.test.js
           node tests/bit-visualizer.test.js
+          node tests/twos-complement.test.js
       - name: Build HTML, EPUB, PDF and Kindle
         run: make all
       - name: Upload course artifacts
diff --git a/.github/workflows/pages.yml b/.github/workflows/pages.yml
index 3257d18..f3a8a1a 100644
--- a/.github/workflows/pages.yml
+++ b/.github/workflows/pages.yml
@@ -58,6 +58,7 @@ jobs:
                 <a href="en/">English</a>
                 <a href="interactive/base-converter/">Basekonverterer</a>
                 <a href="interactive/bit-visualizer/">Bit-visualisering</a>
+                <a href="interactive/twos-complement/">Two’s complement</a>
               </nav>
             </main>
           </body>
diff --git a/interactive/twos-complement/index.html b/interactive/twos-complement/index.html
new file mode 100644
index 0000000..6269994
--- /dev/null
+++ b/interactive/twos-complement/index.html
@@ -0,0 +1,10 @@
+<!doctype html><html lang="no"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1">
+<title>EduNumbers – Two's complement</title><link rel="stylesheet" href="style.css"></head><body><main>
+<p><a href="../../no/">← EduNumbers</a></p><h1>Two’s-complement explorer</h1>
+<p>Samme bitmønster kan bety forskjellige tall. Utforsk signed og unsigned tolkning side om side.</p>
+<section class="controls"><label>Signed verdi <input id="signed" value="-1" inputmode="numeric"></label>
+<label>Bredde <select id="width"><option>8</option><option>16</option><option>32</option></select> bit</label><p id="error" role="alert" aria-live="polite"></p></section>
+<section class="cards"><div><h2>Bitmønster</h2><code id="binary"></code></div><div><h2>Hex</h2><code id="hex"></code></div>
+<div><h2>Unsigned</h2><code id="unsigned"></code></div><div><h2>Signed</h2><code id="signedOut"></code></div></section>
+<section><h2>Hva skjer?</h2><p id="explain"></p><p><strong>Signed område:</strong> <code id="range"></code></p></section>
+</main><script src="twos.js"></script></body></html>
\ No newline at end of file
diff --git a/interactive/twos-complement/style.css b/interactive/twos-complement/style.css
new file mode 100644
index 0000000..a257b44
--- /dev/null
+++ b/interactive/twos-complement/style.css
@@ -0,0 +1,4 @@
+:root{font-family:system-ui,sans-serif;line-height:1.55;color-scheme:light dark}body{margin:0}main{max-width:60rem;margin:3rem auto;padding:0 1.25rem}
+.controls{display:flex;gap:1.5rem;flex-wrap:wrap}.controls label{display:grid;gap:.3rem}input,select{font:inherit;padding:.5rem}#error{flex-basis:100%;min-height:1.5em;font-weight:600}
+.cards{display:grid;grid-template-columns:repeat(auto-fit,minmax(12rem,1fr));gap:.75rem}.cards div{border:1px solid #8888;border-radius:.6rem;padding:1rem}.cards h2{font-size:1rem;margin-top:0}
+code{font-family:ui-monospace,monospace;overflow-wrap:anywhere;font-size:1.05rem}a{color:inherit}
\ No newline at end of file
diff --git a/interactive/twos-complement/twos.js b/interactive/twos-complement/twos.js
new file mode 100644
index 0000000..2bda7d5
--- /dev/null
+++ b/interactive/twos-complement/twos.js
@@ -0,0 +1,20 @@
+"use strict";
+const signedEl=document.querySelector("#signed"),widthEl=document.querySelector("#width"),errorEl=document.querySelector("#error");
+function limits(w){const half=1n<<BigInt(w-1);return {min:-half,max:half-1n,mod:1n<<BigInt(w)}}
+function encodeSigned(n,w){const l=limits(w);if(n<l.min||n>l.max)throw new Error(`Verdien må være mellom ${l.min} og ${l.max}.`);return n<0n?l.mod+n:n}
+function decodeSigned(bits,w){const l=limits(w);return bits>l.max?bits-l.mod:bits}
+function groupedBinary(bits,w){return bits.toString(2).padStart(w,"0").replace(/(.{8})(?=.)/g,"$1 ")}
+function render(){
+ const w=Number(widthEl.value),l=limits(w);let n,bits;
+ try{if(!/^[+-]?\d+$/.test(signedEl.value.trim()))throw new Error("Skriv inn et signed heltall.");n=BigInt(signedEl.value.trim());bits=encodeSigned(n,w);errorEl.textContent=""}
+ catch(e){errorEl.textContent=e.message;return}
+ document.querySelector("#binary").textContent=groupedBinary(bits,w);
+ document.querySelector("#hex").textContent="0x"+bits.toString(16).toUpperCase().padStart(w/4,"0");
+ document.querySelector("#unsigned").textContent=bits.toString();
+ document.querySelector("#signedOut").textContent=decodeSigned(bits,w).toString();
+ document.querySelector("#range").textContent=`${l.min} … ${l.max}`;
+ document.querySelector("#explain").textContent=n<0n
+ ? `Negativ verdi: 2^${w} + (${n}) = ${bits}. Det er det lagrede unsigned bitmønsteret.`
+ : `Positiv verdi: signed og unsigned har samme numeriske verdi så lenge signbiten er 0.`;
+}
+signedEl.addEventListener("input",render);widthEl.addEventListener("change",render);render();
diff --git a/tests/twos-complement.test.js b/tests/twos-complement.test.js
new file mode 100644
index 0000000..7312d0c
--- /dev/null
+++ b/tests/twos-complement.test.js
@@ -0,0 +1,10 @@
+const fs=require("fs"),vm=require("vm");const s=fs.readFileSync("interactive/twos-complement/twos.js","utf8");
+const a=s.indexOf("function limits"),b=s.indexOf("function render"),box={};vm.createContext(box);vm.runInContext(s.slice(a,b),box);
+const eq=(a,b,m)=>{if(a!==b)throw new Error(m+": "+a+" != "+b)};
+eq(box.encodeSigned(-1n,8),255n,"-1/8");eq(box.decodeSigned(255n,8),-1n,"FF/8");
+eq(box.encodeSigned(-128n,8),128n,"-128/8");eq(box.encodeSigned(127n,8),127n,"127/8");
+eq(box.encodeSigned(-32768n,16),32768n,"-32768/16");eq(box.decodeSigned(65535n,16),-1n,"FFFF/16");
+eq(box.encodeSigned(-2147483648n,32),2147483648n,"INT32_MIN");
+eq(box.groupedBinary(0xFF00n,16),"11111111 00000000","grouping");
+for(const [n,w] of [[-129n,8],[128n,8],[32768n,16]]){let ok=false;try{box.encodeSigned(n,w)}catch{ok=true}if(!ok)throw new Error("Expected range rejection "+n)}
+console.log("Two's-complement tests passed");
\ No newline at end of file