Hi everyone, first post here. Looks like a great forum and happy I found it.

I'm in the midst of installing a bathtub, and for reasons I can't explain, I measured only once (ugh) and cut the brass tail (drain?) pipe too short. Can I extend the pipe by soldering on a coupler/union, and if so, what kind of coupling am I looking for? I purchased a 1-1/2" copper coupling for the bath drain pipe (1-1/2" OD) but it's entirely too large...clearly I am missing some very basic knowledge here. A short length of brass pipe with threaded ends that could be used with slip nuts would also work, but I don't know if such a fitting exists. Or do I need to buy a whole new assembly because of my stupidity?

Sorry for what I am sure is a very naive question - I'm an advanced DIYer in everything BUT plumbing. I appreciate the help!