tt is an implementation of the well-known game tetris.  Quadominoes (groups
of four squares joined orthogonally together) slowly fall down the screen,
accumulating at the bottom, and when the pile reaches the top of the screen
the game is over.

The pieces may be moved to the left or right, and rotated as they fall, with
the aim of making them tessellate with the pieces already at the bottom of
the game area.  The height of the stack of pieces can be reduced by filling
a complete row of ten squares, at which point that row will disappear, and
those above will fall down into its place.  It is possible (and desirable)
to destroy multiple rows at once.

WWW: http://www.miketaylor.org.uk/tech/tt/