This article discusses an important aspect of the law relating to theft in thirteenth-century England, and one of the ways in which that law developed. Central to it is the argument that the treatise The Mirror of Justices and references in court records and reports show that a short statute enacted early in the reign of Edward I, probably in 1278, categorically defined 12d. as the amount, whether in goods or money, at which larceny became a capital felony, incurring judgment of death. As well as setting out the evidence for this hitherto overlooked ordinance, the article also argues that the statute can be associated with some significant developments in the way petty theft was treated subsequently. In particular it had the effect of promoting the development of penal imprisonment, while since the task of valuation was given to trial juries, it further enhanced the leading role of the latter in determining the fates of the men and women whose lives depended on their verdicts.