5x5 square board; 9 black and 9 white pieces.
Black starts, placing a piece anywhere on the initially empty board.
White then does the same.
Thereafter, each player's move has two parts: move a piece two or more in any unobstructed line, orthogonally or diagonally, then drop a piece on the last square passed over. If all 9 pieces have been placed, the player first makes the move and then picks up any of his already-dropped pieces and drop it behind the moved piece.
For example, on an empty board, a piece on a1 could move to a3 (dropping a piece in a2) or a4 (dropping on a3) or d4 (dropping on c3) or e5 (dropping on d4).
Exception: to counterbalance going second, the second player may simply place a piece (rather than moving and dropping) on his second move.
The object is to get four (or five) pieces in a row, orthogonally or diagonally, or to block the opponent from moving.