These are the finest doors made. Precision in every aspect of the manufacturing process gives you a door that will seal every time and take years of abuse yet remain beautiful and working perfectly.  The Single Flap model is designed for customers in moderate climates that may be more concerned with bugs etc. that may intrude. On the Single Flap model; the flap is positioned towards the exterior of the opening for optimal weather protection (Unlike other cheaper models).  The dual flap models are the most popular as they provide a nearly perfect seal to the outdoors. They provide a superior insulating barrier and protection from weather and temperature.  One flap is mounted to the inside, one is mounted to the outside; each swing and seal independently and perfectly.