What is "modus decimandi"

Modus \Mo"dus\, n.; pl. Modi. [L. See Mode.] (Old Law)

1. The arrangement of, or mode of expressing, the terms of a contract or conveyance.

2. (Law) A qualification involving the idea of variation or departure from some general rule or form, in the way of either restriction or enlargement, according to the circumstances of the case, as in the will of a donor, an agreement between parties, and the like.
   --Bracton.

3. (Law) A fixed compensation or equivalent given instead of payment of tithes in kind, expressed in full by the phrase modus decimandi.
   --Blackstone.

   They, from time immemorial, had paid a modus, or composition.
   --Landor.