Both styles of pockets will fit one piece molded plastic chairs and the 2 piece chairs. It is just a matter of getting the correct size. If you have the molded plastic chairs, measure the across the front of the chair back about 6" down from the top, and also where it meets the seat (this will probably be the same measurement as the seat width).
If you have the 2 piece chairs, measure the seat back across the widest part. You can measure across the front, unless there are metal bars across the back of the chair's back...then measure across the back as the bars take up room.
Here are some examples of sizes and chairs:
12" swing back will fit the one piece molded plastic chairs that measure:
9" at the middle
10" at the seat
14" regular style and 13" or 14" swing back style will fit 1 piece molded chairs that measure :
8"-9 1/2 at the top
9 1/2 " - 10 1/2" at the middle
12" at the seat
16" regular and 14" swing back fit 1 piece molded plastic chairs measuring:
10" at the top
12" in the middle
14" at the seat
and the 2 piece chairs measuring:
13" across the front or 14" across the back