Assach, in Welsh law, was an ancient form of compurgation used in Wales. A person on trial was allowed to call in 300 compurgators, including neighbors and acquaintances, who would swear that the accused person was speaking truth in making oath of his innocence.
A statute from 1413 (I Henry V., c. 5), refers to the then late rebellion in Wales and complains that the Welshmen are still taking revenge for the deaths of their kinsmen against the king's faithful lieges. Some of such lieges they keep in prison until they have paid ransom, or until they have purged themselves of the death of the said rebels,
- "...par un assach selonc la custume de Galles, cest a dire par le serement de CCC hommes."
- "...by an assach according to the custom of Wales, that is to say by the oath of 300 men."
While the Welshmen were doing as they wished, they were still in part acting according to their notions of law by requiring three hundred compurgators.
- This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "article name needed". Cyclopædia, or an Universal Dictionary of Arts and Sciences (first ed.). James and John Knapton, et al. 
- Fisher, H.A.L. "The Laws of Wales". The Collected Papers of Frederic William Maitland. Retrieved 2006-01-27. 1911 edition.