Unlike banks which are open to the general public, membership of credit unions in the UK is restricted to those who fit a defined 'common bond', which is legally defined in their rules.
A common bond is a set of qualifications for membership. This might include those living or working within a particular city, borough or postcode, or those who work for a particular employer or in a certain industry.
Traditionally, credit unions are based on the principle of trust between their members. Because of this, the idea is that all members of a particular credit union should share something in common with one another, —such as being neighbours and colleagues.
There are hundreds of credit unions in the UK, each of which serves a particular community and has a common bond of its own.