A manuscript of practical advice written by the Heidelberg master Lorenz Lechler for his son: how to proportion a vaulted choir (walls, buttresses, windows, ribs and mouldings) from a single measure, the width of the choir.
Lechler worked in the Palatinate at the end of the Gothic period, and wrote his Unterweisung in 1516 as a father’s instruction to a son entering the trade. The original is lost; the text survives in a later copy. Mixed with the technical rules is a good deal of moral advice about how a master should behave: to be modest, to keep faith with his patrons, and not to build beyond his skill.
The technical core is a system of proportion for a single-vessel vaulted choir. Everything is referred to the clear width of the choir. As the text is usually read, the thickness of the wall is a tenth of that width; the buttresses, window jambs, mullions and ribs are then given as simple multiples or fractions of the wall thickness, and the heights as multiples of the width. Lechler also gives templates for the cross-sections of ribs and mullions derived from the same measure.
Rules of this kind (structure proportioned to span) were how Gothic builders achieved safety without calculation. Modern analysis of masonry has shown why they work: a vault that stands at one size will stand at any size if every part is scaled with it, because stresses in masonry are low and stability is a matter of geometry.
The book, part by part
I The choir and its walls
The width of the choir as the governing measure; the wall a tenth of it; buttresses and window openings in multiples of the wall.
II Ribs, mullions and templates
The sections of ribs and window mullions drawn from the wall thickness, with advice on setting out the templates (Brettstücke) from which the stones are cut.