A key field that can be a user-defined character. Btrieve allows two types of null keys: any-segment (called a manual key in earlier versions of Btrieve) and all-segment (simply called a null key in earlier Btrieve versions).

An any-segment null key does not include a particular record in the index if the value of any key segment of that record matches the null value.

An all-segment null key only excludes a particular record from the index if the value of all key segment of that record matches the null value.