| Possible Combinations | |||||
| USMARC | UTILITY | EXPLANATION | |||
| 4xx | nnnn | BLANK | Simple cross reference | ||
| Do not supply any }w values | |||||
| nnan | nna | Linking reference to a pre-AACR2 | |||
| heading (optional) | |||||
| nnaa | nnaa | Pre-AACR2 heading is not a valid | |||
| AACR2 reference | |||||
| nnen | nne | Valid cross reference was also an | |||
| earlier AACR2 heading | |||||
| nnnb | nnnb | Reference not displayed; 664 | |||
| field used (may find but never use) | |||||
| 5xx | nnnn | BLANK | Simple see also reference | ||
| Do not supply any |w values | |||||
| annn | a | Earlier heading | |||
| bnnn | b | Later heading | |||
| nnnc | nnnc | See also reference for | |||
| pseudonyms (663 field used) | |||||
| nnna | nnna | Reference not made (displayed) | |||
| bnna | bnna | cf. RI26.3B-C(1b), p.2 | |||
| anna | anna | (unlikely ever to use) | |||