index usage in joins q'n

http://www.postgresql.org/docs/8.2/interactive/indexes-intro.html
states that "Indexes can moreover be used in join searches. Thus, an
index defined on a column that is part of a join condition can
significantly speed up queries with joins."

Does this mean that a condition like "WHERE ... [AND] lhs.a=rhs.b [AND]
..." where rhs.b is already unique-indexed, also requires (non-unique)
index on lhs.a for maximal join speed? Otherwise why would they want to
say that?

Thanks.