This part is a (1) 1" Slip X 1" Slip Coupler or Coupling PVC Plumbing Fitting. Used to connect two pieces of pipe or a fitting.

The Coupler receives a 1" fitting or pipe which measures 1 1/4" OD on one side and then receives a 1" fitting which measures 1 1/4" OD on the other side.

The inside diameter of each slip side is is 1 1/4".

Note: The actual size name of a PVC part is not the actual size of the hole in the part, it is really the Inside Diameter of the pipe that would glue into that part, which doesn't make sense, since you cannot see inside the pipe. Therefore I am giving you the measurements of the outside of the pipe so you know what size part to order. For example a PVC part that is called 1/2" (Which meassures 3/4" OD) would glue in to a fitting that measures 3/4" ID.

This is a chart for the outside of your plumbing pipes so you can figure out what size parts to order

Pipe Outside Diameter chart:
½” Outside Diameter is ¾”
¾” Outside Diameter is 1”
1” Outside Diameter is 1 ¼”
I have never seen 1 ¼” in Hot Tubs
1 ½” Outside Diameter is 1 ⅞”
2” Outside Diameter is 2 ⅜”
2 ½” Outside Diameter is 2 ⅞”