"Swap" variant became very popular thanks to the option of side and colour swapping (taken from Renju and slightly modified). The first moves are the following:
Starting player places two own (crosses) and one opponent's (circle) piece to the board. These moves can be done anywhere, it is not mandatory to begin at the board centre.
The opponent (black, circles) chooses either to continue in the game in a standard way (places a circle) or swaps the colours. The latter choice means that the black becomes the white (and vice versa) and the former white (current black) performs a normal move placing a circle.
After the sides and colours are definitely set, the game continues with classic Five in Line rules (except one detail, see the next point).
Important: A player must place exactly five pieces in a solid line to win! Making a row of six, seven or more pieces does not finish a game.