Features inner telescoping frame that adapts to doors of 1-1/4" to 1-3/4" thickness. Impact-resistant super-strong thermoplastic. Clear flap allows pet to see if slide is in place. Tamper-proof mounting, door cannot be removed from outside. Easy installation. How to select the right pet door: Step 1: Measure your pet from the top of the shoulders to the chest, just behind the front legs. Step 2: Determine your pet’s weight. Step 3: Select the correct door size based on your pet’s measurements and weight. No. PPDSL: Thickness: 1-1/4" to 1-3/4", Flap Dimension: 15″ × 20″, Cut-Out Dimension: 16-5/8″ × 22-3/4", Outside Frame Dimension: 18″ × 24″, Dog Size: Super Large, Material: Thermoplastic