Copyright (c) 2003 Joćo Neto

UN is played on the following board:

SETUP - One player drops the neutral stone on a cell and the other chooses who starts.
TURN - On each turn, each player slides the neutral stone a number of empty cells in a straight line and each cell passed becomes unavailable for the rest of the game.
GOAL - The first player that cannot move loses.
An example

The neutral stone was dropped in the middle cell (the marked white stone). The numbers indicate the game moves.

An example

In this position, the next player loses the game. Whatever he does, the other player will stalemate him (moving to one of the red dots). 

Bill Taylor and Richard Rognlie extended the concept to square boards (with diagonals, which gives the game a greater depth) and to multiple heads (increase the tactical element). The idea behind multiple heads is that each sliding will create a new head until a maximum number is achieved. Bill called these multiple head variants Hydra for hexagons and Medusa for squares. Un, in this context, is Hydra(1). Here is an opening example of a Hydra(4) game:

4 heads

A position of Hydra(4) after six moves. The first four moves created a new head. Now each player must manage those heads until all are stalemated.

Related games: Slimetrail and Snailtrail.