أحدها: دلالة العرف
The first ground is the indication of custom.
فإذا كان المعنى متعارفاً بين الناس كان ذلك المعنى المتعارف دليلاً على أنه هو المراد به ظاهراً
A meaning established among people is prima facie evidence that the speaker intended it.
An expression may have a broad lexical meaning but a narrower established use in transactions and oaths. The generally understood usage then helps determine what the speaker meant.
For example, *raʾs* means 'head' generally, but the customary expression 'buying a head' may refer to heads normally traded as such. A literal possibility outside that usage is not automatically brought within the oath.
A claimed custom must actually be recognizable, rather than merely a private or idiosyncratic usage.