diff --git a/package.json b/package.json
index dade2c1..79e1c3f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "csset",
-  "version": "0.0.4",
+  "version": "0.0.5",
   "description": "experiment to perform algebra of sets with css",
   "main": "./lib/index.js",
   "types": "./lib/index.d.ts",
diff --git a/src/csset.spec.ts b/src/csset.spec.ts
index 7313acb..b3e7ba9 100644
--- a/src/csset.spec.ts
+++ b/src/csset.spec.ts
@@ -380,4 +380,49 @@ describe('Csset', () => {
       });
     });
   });
+
+  describe('union', () => {
+    test('should return one set if is superset of the other', () => {
+      const data = [
+        {
+          sel1: 'div, p, aside, section',
+          sel2: 'div, p, section.class',
+          expected: 'div,p,aside,section',
+        },
+        {
+          sel1: 'div, p, section.class',
+          sel2: 'div, p, aside, section',
+          expected: 'div,p,aside,section',
+        },
+      ];
+
+      data.forEach( d => {
+        const set1 = new Csset(d.sel1);
+        const set2 = new Csset(d.sel2);
+        const result = `${set1} ${operationSymbols.union} ${set2} => ${set1.union(set2)}`;
+        const expected = `${set1} ${operationSymbols.union} ${set2} => ${d.expected}`;
+
+        expect(result).toEqual(expected);
+      });
+    });
+
+    test('should remove rules that are subset of others', () => {
+      const data = [
+        {
+          sel1: 'div, p, aside, section.class',
+          sel2: 'div, p#id, span, a, section',
+          expected: 'div,p,aside,span,a,section',
+        },
+      ];
+
+      data.forEach( d => {
+        const set1 = new Csset(d.sel1);
+        const set2 = new Csset(d.sel2);
+        const result = `${set1} ${operationSymbols.union} ${set2} => ${set1.union(set2)}`;
+        const expected = `${set1} ${operationSymbols.union} ${set2} => ${d.expected}`;
+
+        expect(result).toEqual(expected);
+      });
+    });
+  });
 });
diff --git a/src/csset.ts b/src/csset.ts
index c73db92..bda07d2 100644
--- a/src/csset.ts
+++ b/src/csset.ts
@@ -86,7 +86,36 @@ export class Csset {
    * @param set the set to check with
    */
   subsetOf(set: Csset): boolean {
-    return set.subsetOf(this);
+    return set.supersetOf(this);
+  }
+
+  union(set: Csset): Csset {
+    if (this.supersetOf(set)) {
+      return this;
+    }
+
+    if (this.subsetOf(set)) {
+      return set;
+    }
+
+    // return new Csset('unknown');
+
+    // If one of the sets does not have subsets just return a set with all
+    if (this.layers || set.layers) {
+      return new Csset(`${this},${set}`);
+    }
+
+    // Make union in subsets if possible
+    const equalSets = this.subsets.filter(thisSet => set.subsets.some(otherSet => `${thisSet}` === `${otherSet}`));
+    const uniqueOne = this.subsets.filter(s => !s.subsetOf(set));
+    const uniqueTwo = set.subsets.filter(s => !s.subsetOf(this));
+    
+    const equSelector = equalSets.map(s => `${s}`).join(',');
+    const oneSelector = uniqueOne.map(s => `${s}`).join(',');
+    const twoSelector = uniqueTwo.map(s => `${s}`).join(',');
+
+
+    return new Csset(`${equSelector},${oneSelector},${twoSelector}`);
   }
 
   toString(): string {