We developed a sturdy thermoplastic (ABS) frame; a see through poly vinyl flap with patented edges to not to warp; a floating sill to insure good closure; and a strong molded ABS lock-out slide to restrict the pet"s movements when necessary. Then we made it 4 sizes for pets up to 120 pounds. And with a tamper proof mounting system and easy installation, it"s the most economical pet door in the industry.