Full Idea
One and the same universal recurs; it is multiply located; it is wholly present in both instances, a shared common part whereby the two instances overlap. Being alike by sharing a universal is 'having something in common' in an absolutely literal sense.
Gist of Idea
Universals recur, are multiply located, wholly present, make things overlap, and are held in common
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.64
A Reaction
A helpful spelling out of the commitment involved (in Armstrong and others) in belief in universals. To me this is a convenient list of reasons why the whole proposal is nonsense. Why does Lewis take them seriously?