Using subselects as joins in POstgeSQL (possible?, examples)

Is it possible to use subselects as joins in PostgreSQL.

eg.

select a.a, a.b, a.c, b.a, b.b. b.c from (first subsselect) a (second subselect)
b where (in table criteria) and a.a = b.a

or

select a.a, a.b, a.c, b.a, b.b. b.c from (first subsselect) a join (second
subselect) b on a.a = b.a where (in table criteria)

I have a feeling it is possible but I need the right syntax

//Frank

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