It is used only when a current publication frequency is given in field 310.

Subfield $a contains the frequency statement without dates of the former publication frequency. Subfield $b contains the date of the former publication frequency.

The following frequency statements might be used in subfield $a.

Annual
Frequency unknown
Frequency varies
Issued every [three] [ weeks, months, years]
Issued every two years
Issued twice a week
Issued twice a month
Issued twice a year
Three issues yearly
Monthly
Quarterly
Weekly

Examples:

310 $a Monthly, $b 1958-1976
[Serial ceased publication in 1976]
321 $a Bimonthly, $b 1954-1957

310 $a Five issues a year, $b 1990-
321 $a Four issues a year, $b 1970-1989