TRANSOM (probably a corruption of Lat. transtrum, athwart, in a boat; equivalents are Fr. traverse, croisillon, Ger. Losholz), the architectural term given to the horizontal lintel or beam which is framed across a window, dividing it into stages or heights. In early Gothic ecclesiastical work transoms are only found in belfry unglazed windows or spire lights, where they were deemed necessary to strengthen the mullions in the absence of the iron stay bars, which in glazed windows served a similar purpose. In domestic work, on account of the opening casements, they are more frequently found. In the later Gothic, and more especially the Perpendicular period, the introduction of transoms became very general in windows of all kinds.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)