Kavua (The Fixed-in-Place Doubt That Is Treated as a 50-50)

Normally, when something is in doubt, Jewish law follows the majority (Rov): if most shops in town sell kosher meat, meat of unknown origin is presumed kosher. But kavua ('fixed in place') reverses this. If the item is doubtful in its fixed location, the doubt is treated as an even 50-50 split (kol kavua k'mechetza al mechetza), as if the kosher and non-kosher sources were evenly matched. This counter-rule to Rov is foundational to the laws of forbidden mixtures and to capital procedure.