Features inner telescoping frame that adapts to doors of 1/2" to 1-3/4" thickness. High-quality aluminum construction. Clear flap allows pet to see if slide isn't in place. Tamper-proof mounting, door can't be removed from outside. Thermo-Insulated Lockout Slide . Available in 3 sizes for pets up to 120 lb. White. Installation measurements is the opening you cut in the door to install. 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.