Composite Keys

[RPK <rohitprakash123(at)indiatimes(dot)com>]

First, can a primary key field be only one? I mean can I define a primary
containing two fields. 

I have a table called "Payments" where there are two fields, "ReceiptNo" and
"PaymentDate". If I take "ReceiptNo" only as a primary key then chances are
of "primary key violation". Because in some places the receipt books have 
numbers that start with 1. I mean Receipt Book 1 nos. 1-100. Receipt Book 2
nos. 1-100, and so on.

So I want to take ReceiptNo and PaymentDate as primary key field. Is it
possible to define composite key in PGSQL. Please also suggest if any other
approach would be fine.
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