Pet door that features inner telescoping frame that adapts to doors of 1/2 In. to 1-3/4 In. thickness. Includes high-quality aluminum construction, and a clear flap to allow pet to see if slide isn't in place. Features tamper-proof mounting so that the door can't be removed from outside and a Thermo-Insulated lockout slide. 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.