I get that the same position can be reached by several diferent move orders. I'm just wondering if the same player doesn't have the move in the position is it still a transposition?
My guess is no, but that is just a guess based on one player having a move in hand so to speak.
Any transposition experts out there who know the answer rather than my puny guess?
If there was a table of FENs with opening names associated with the positions, would you just need to compare the first field (piece placement) or the first and second (active player) to see if a name for it is in the table? The first and second seem to be my feeling here, but as I said I'm just guessing.
1. d4 Nf6 2. c4 e6
1. c4 Nf6 2. d4 e6
1. d4 e6 2. c4 Nf6
1. c4 e6 2. d4 Nf6
These are all valid transpositions to reach what could become either the NimzoIndian or the Catalan (depending on whether 3. Nc3 or 3. g3 is played).
1. d3 Nf6 2. d4 e6 3. c4
Is not a transposition as above as the position is different (a position includes information such as which side is to move, en passant and castling availability).
So you have to check all of; the piece distribution, the side to move, castling eligibility (temp, permanent?) and en passant.