Twelve men connected with a large firm in the City of London sit down to
luncheon together every day in the same room. The tables are small ones
that only accommodate two persons at the same time. Can you show how
these twelve men may lunch together on eleven days in pairs, so that no
two of them shall ever sit twice together? We will represent the men by
the first twelve letters of the alphabet, and suppose the first day's
pairing to be as follows--
(A B) (C D) (E F) (G H) (I J) (K L).