As well as the envelope and the stamp, an integral part of a First Day Cover is the postmark or handstamp.
These can be divided into four different types: special handstamps, ordinary first day of issue postmarks, counter date stamps (CDS) and slogan postmarks.
These are specially designed postmarks which are very popular with collectors. Nowadays they are applied by special machines which give better impressions and allow for more detail but up until the 1990's they were applied by handheld stamping machines.
Ordinary First Day of Issue (FDI) Handstamps
These are circular postmarks provided by the Royal Mail and became more common from the 1960s when the GPO recognised a need for proper philatelic handstamping
Counter Date Stamp (CDS) Postmarks
Sometimes called circular date stamps, these are the everyday cancellations that are available at all Post Offices.
If such a postmark had any connection with an issue was just luck. During the 1960's up to the present day, collecting First Day Covers with relevant or appropriate CDS postmarks has become of added interest to collectors.