Double sink rough in. . .

I use tubular 1.5| p-traps that some with a reducing slip joint washer 1.50" x 1.25"
I buy those by the dozen, complete with the 1.50" trap adapters.

Thanks, Terry.

I'll end up with something like this.

Is a washer not necessary at the threaded adaptor? Or is that not the correct adaptor? Do I, perhaps, need a different nut than what came with it? The washer works when using one of the nuts from the trap kit. I'm assuming I need one of those slip joint nuts?

In that picture you posted, the slip joint nut has the slip joint washer incorporated as an "all in one".

Ah. I thought that might be the case. I edited that out of my comment...I didn't want to sound stupid! . I see what your talking about there.

Thank you once again.

Tom