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bi-, semi-

Forms solid compounds except when followed by a capital letter or i, or when unreadable.

To avoid confusion, the following uses are strongly recommended:
            semiweekly or better twice a week
            biweekly or better every two weeks or every other week
            semimonthly or better twice a month
            bimonthly or better every two months or every other month
            semiannual or better twice a year
            biennial or better every two years or every other year