Three-way plastic register side wall/ceiling air register with multi-shutter damper in white (6" x 12") duct opening size) Size Selection Tips: 1. Measure the duct opening size. 2. Choose the register size that equals to the duct opening. 3. Outside measurement will be 2 larger in order to cover the hole. 4. DO NOT choose the one for outside measurement, it will be TOO BIG to fit in.