Top "H" Section Spring Loaded Bar and Adjustment Knob for Aluminum Pet Patio Doors.
To determine the correct size door simply measure the width (A) and height (B) of your pet.
(A) Measure the widest point of your pets shoulder
(B) Measure from the top of your pets back (just behind his head) to the bottom of chest.
FLAP SIZE DETERMINES ALL REPLACEMENT PART AND PET DOOR SIZES: 5"x 9-1/4"(S), 7-1/4"x13"(M), 9-3/4"x17"(L/XL), 15"x23-1/2"(XLG/SL)