When a player plays his first piece, does it have to cover the corner square? For example, can one play his X piece as first piece?
According to the rues, "The first piece played by each player must cover a corner square." They show an example of an illegal first move, as a piece that "surrounds" the corner without touching it, like an X would. This does mean that the X cannot ever be the first piece.
The X is the only piece which cannot be played first.
There is also a smaller (14x14 board) version of the game for 2 colors, called Duo, or sometimes Travel Blokus, and the starting point for that is 5 squares in from the corner on the diagonal.
In that game any piece can be played first, and the X is often chosen.