In one of the cabinets, we see a plumb bob and set square hanging from a peg as well as a sandglass, cittern, and pair of dividers. The grouping of these objects may at first seem puzzling but becomes clear once we realize that all five are concerned with measurement and proportion. The set square and plumb bob were used as a level by builders and architects. The dividers were employed to measure distances on a chart or to determine the scale of a design. The sandglass, an instrument popular in the late Middle Ages, indicated the equal hours, which came into general usage in the last third of the fourteenth century. During the fifteenth century the theory of musical proportions and the studies of linear perspective and architectural proportions came to be seen as expressions of the same mathematical truth, so even musical instruments such as the cittern depicted here must be understood as an allusion to the theory of music's harmonic proportions. Following Pythagorean teaching, according to which musical intervals correspond to the arithmetic ratios of the length of a string, music was considered a science in antiquity and included among the liberal arts in the medieval era.

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