Customs are well-tested in diverse social situations and, as a result, they are efficient, sustainable, and practical. Their conversion into a customary legal system, however, requires a thorough understanding of their socioeconomic and cultural background. Otherwise, customary laws can become rigid, impractical, and sometimes unreasonable. This article discusses the close relationship between customs and customary law in the Kandyan region of Sri Lanka and the risks embedded in the interpretation and application of such laws in the current socioeconomic and cultural milieu. The article discusses a recent judgment of the Supreme Court of Sri Lanka that dealt with customary marriage and inheritance rights in the Kandyan region and held that a _diga _(patrilocally) married woman forfeits her inheritance rights to her father’s intestate property. The article argues, using the findings of anthropological studies in the Kandyan region, that the judgment was incompatible with some of the Kandyan customs. In the absence of a codified legal system, at least an Interpretation Guide on Kandyan customary law is needed to help avoid such incompatibilities. Anthropologists and sociologists can provide a socioeconomic and politico-cultural context for the Guide.