The signs of the zodiac paired with the labours of the months are common in medieval art. Countless variations appear in sculpture, illuminated manuscripts, stained glass and other media. A clear understanding of the oddities of two particular variations at Chartres Cathedral has long been elusive and problematic. The variation in the Royal Portal and another in the Zodiac Window are peculiar for two reasons. First, the sculptures are bi-directional in their calendrical sequencing with a disconnected placement of Gemini and Pisces. Second, the otherwise conventional calendrical sequence in the window transposes the placement of the May/Gemini pair with the April/Taurus pair. Considering that the sculpture and the glass both have an anomalous treatment of Gemini, this redundancy may be approached as a deliberate design decision made with the intention of conveying specific deeper meaning that would otherwise not have been possible. The hypothesis is that it may be understood as a visual metaphor intended to designate a biblically estimated duration of the Old Testament period.