Assuming it's a concert grand and you are recording - I use a pair of SDC omnis (Gefell M221, Neumann KM131-D, Sennheiser MKH8020 or MKH20 are what I would use).
I set these about 20cm apart at about ear height and about 2-metres from the piano.
This is my "go-to" starting position.
I will vary this depending on the piano, the room, the pianist and the musical work.
Depends on the type of piano (grand vs. upright), whether this is for recording or sound reinforcement purposes, whether you're trying to get an up-close sound like you might use in a rock mix or a more traditional sound for classical recordings (with the mic often placed a fair distance away in a good-sounding space), etc.