Storage of Foreign Keys

Hi,

can anybody tell me how postgres internally stores foreign keys...
E.g. I have a table "tableA" with column "mycolA text" and the constraint
FOREIGN KEY(mycolA) REFERENCES tableB(mycolB)

Is everything I insert into mycolA and mycolB stored twice, or
is there something like a pointer/reference from mycolA to the things
stored in mycolB?

Thanks alot for any information, Christian