It has been suggested by Franciscus Junius that the modern Westfrisian collocation binnen de balken (‘inside the beams’, i.e. in the house, indoors) goes back to an old legal term. This article deals with four possible explanations for the origin of this collocation, three of them being in a legal sphere. The explanation that Piirainen (2006) suggests for the variant in the Westmunsterland dialect (‘inside the four walls’), seems to be the most plausible one, because it leaves fewest questions. It may be applicable for the Westfrisian situation too. So, the Oldfrisian legal term binna wagum etc. could be the direct predecessor of the modern Westfrisian collocation binnen de balken.