Need indexes on inherited tables?

Dear Postgresql experts,

I have a base table that declares a primary key spanning a couple of columns:

create table B ( id integer, xx someothertype, ..... primary key (id, xx)
);

and a number of derived tables that inherit from B:

create table T (....
) inherits (B);

An index is automatically created for B because of the primary key.

If I search for something in T using the key columns, e.g. I do
 select * from T where id=1 and xx=something;

will the index be used?  Or must I explicity create an index on id and xx for T and each of the other derived tables?

Is it any different if I search in B and find rows that are actually in T?

(Slightly unrelated: does the index on (id,xx) help when I am searching only on id?)

Thanks for any insight anyone can offer.

--Phil.