from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
PRINCIPAL OBLIGATION. That obligation which arises from the principal object
of the engagement which has been contracted between the parties. It differs
from an accessory obligation. (q.v.) For example, in the sale of a horse,
the principal obligation of the seller is to deliver the horse; the
obligation to take care of him till delivered is an accessory engagement.
Poth. Obl. n. 182. By principal obligation is also understood tho engagement
of one who becomes bound for himself and not for the benefit of another.
Poth. Obl. n. 186.