Telescoping fitting. This fitting glues into a 1.25" Fitting (like a couple) and allows 3/4" Sch 40 sized pipe (1.05" OD) to slide into the 1.25" pipe. If you use a fitting like a couple, the other side of the couple will take the 1.25" pipe. If you want to stabilize the two pieces of pipe you'll want one on either and and/or one on the end and one in the middle depending on your application. If you do this you'll need to round off or chamfer (bevel cut) the end of the 3/4" pipe so it goes smoothly into the sleeve.
You can also use these in a Tee fitting for some interesting projects. See this video.
