A fun little retro

During a search for another problem in another thread, I came upon this little problem:

What was the last move?

(If your name is Remellion, chaotic_iak, caveatcanis, or shoopi, please don't post the answer in the thread, but rather message it to me. I'd like to give some of the less-experienced solvers a crack at this one.)

If white played last, they did axb8=R+ or Rd7. Black would be in check it it were Rd7, so Rd7 is impossible.

If black played last, it's illegal since the current position is check.

axb8=R+ is the answer.

Edit: I just realized this is quite old. Whoops.

By "last move", you need to also figure out what got captured. axb8=R+ is indeed the move when written in algebraic notation, but what was the unit on b8?

Also, yes, this is quite old, but I suppose it's better to at least add something to the thread than bumping without reason.

If it was a knight, white would be in check prior to the position.

If it were a rook, how did the king get to c8? The rook must have gone through the 8th rank, but that's impossible because of white's two rooks.

If it were a bishop, how did the pawn get to a7?

It must be a queen. Or at least that's what I got.

i apologize for bringing up such an old thread, but how do you even figure out what piece was on b8? I understand why it cannot be a knight, but I can't understand why it would be other pieces.

There is only one way to find out and that is by taking back moves. To solve retrograde problems you need to learn to play backward chess as agile as you play forward chess. When you can't go back further (named retro-stalemate) you made an earlier mistake which you have to correct first. You may stop when you feel the rest is "easy".

Btw, the 3rd post gives the solution!