| Frequency | Note | Regularity |
| b=blank | Irregular | x |
| a | Annual | r |
| b | Bimonthly | r |
| c | Semiweekly | r |
| d | Daily | r |
| e | Biweekly | r |
| f | Semiannual | r |
| g | Biennial, every 2 yrs | r |
| h | Triennial | r |
| i | Three times a week | r |
| j | Three times a month | r |
| m | Monthly | r |
| q | quarterly | r |
| s | Semimonthly | r |
| t | Three time a year | r |
| u | Unknown frequency Unknown irregularity |
u |
| w | Weekly | r |
| z | Other: Frequency varies | x |
| a-w | Normalized irregular - irregular in predictable pattern | n |
Frequency codes should agree with the Regl code. Notes in fields 310 and 321 should provide text.
Example: Freq:
m
Regl.: x
310: Monthly
321: Frequency varies, |b 1948-1984
Freq: q
Regl: r
310: 4 issues yearly, |b Aug. 1904-
321: Five issues yearly, |b 1901-June 1904
Freq: m
Regl: r
310: Monthly, |b 1949-1956
321: Five issue yearly, |b 1947
321: 14 issues yearly, |b 1948
* Ceased publication: If the serial has ceased publication, use the latest frequency in Freq.