Originally posted by cmsMaster
I still don't get it... 🙁 many openings have Bc4 in them, but that isn't considered transposition by any means.
Transposition simply means that one series of moves gives the SAME POSITION at the end of the move sequence as a second series of moves. Because one series of moves has been given a name eg Bishop's Opening (1.e4 e5 2.Bc4) and the second eg The Vienna
(1.e4 e5 2.Nc3) we say that The Vienna can transpose into the Bishop's opening. This can occur after the sequence 1. e4 e5 2. Nc3 Nf6 3. Bc4 which gives the same position as 1. e4 e5 2. Bc4 Nf6 3. Nc3...
In other openings the Caro Kann can transpose into a Semi-Tarrasch or Nimzo Indian and so on. It is just more convenient to use the names of openings than type out a list of moves when trying to discuss them.
Bc4 does occur in many openings eg Guioco Piano, Sicilian Sozin, etc but those move orders that have been given other names, and lead to positions that do not necessarily arise from Double King Pawn Openings such as the Vienna and Bishops Opening.
None of these conventions of theory really matter unless you start to try to study openings and need to know what it is called to find it in a book or a database, or discuss it with other players and need a common language to do so.