d	daily	once per day
i	--------	three times per week
c	semiweekly	two times per week ('half of a week')
w	weekly	once per week
j	--------	three times per month
e	biweekly	every two weeks
s	semimonthly	two times per month ('half of a month')
m	monthly	once per month
b	bimonthly	every two months
q	quarterly	four times per year
t	--------	three times per year
f	semiannual, *biannual	two times per year ('half of a year')
a	annual	once per year
g	biennial	every two years
h	triennial	every three years
z	--------	other frequencies (quadrennial - every four years, quinquennial - every five years, etc.)
k	irregular	known to be so (no determinable frequency)
u	--------	unknown frequency

* sounds contradictory, but semiannual and biannual are used interchangeably

Sources:

CONSER Editing Guide, 1994
ISDS Manual, 1983
MARC 21 Format for Bibliographic Data, 1999

3/2/00 PC