This photograph shows the jig we invented for making the sticks to mark out the lateral spacing of the strings.
A walnut block was made with a V notch in it. All of the marking-out sticks were made together at the same time and they were all made of precisely the same size. A slot for the stick was made in the walnut block at right angles to the V notch. The bottom of the V notch was made so that the top edge of the stick just protruded into the space of the notch so that the measured nicks in the stick could be cut out using the V notch.
The octave span of the strings and keyboards was made (more-or-less arbitrarily) equal to 7½ once. The 3-octave span was therefore 3 x 7½ x 21.834 = 491.3mm. This is close to the 3-octave span of many historical instruments and very little different from the 3-octave span of the modern piano, and is therefore comfortable for most modern keyboard players. The space occupied by each of the 12 notes in one octave is therefore 1/12 x 7½ x 21.834mm = 13.64625mm.
The jig is being used there to mark out the lateral string-spacing stick. A vernier scale was made by dividing 9mm into 10 equal spaces and placed on the block in order to space each nick accurately to the nearest tenth of a millimetre. This therefore gives a very regular and accurate spacing for the strings with 12 strings or one octave occupying a space of precisely 7½ once.