Ratio Table and the Alternate Proportion (Series 161)
Paired magnitudes e g, a c, b d, f h, with the instruction to use permutando
Series 161 pairs magnitudes — e 12 with g 24, a 4 with c 6, b 2 with d 3, and f 6 with h 12 — and closes with the direction that the alternate proportion (la 'mpromutata, permutando) is to be employed. Two faint line-segments with interior division points carry the values across the upper part of the sheet.
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Series 161 and the alternate proportion
The table sets e 12 against g 24, a 4 against c 6, b 2 against d 3, and f 6 against h 12, then instructs that the permutation (alternate proportion) be applied. Alternating the terms exchanges the middle magnitudes while preserving the ratio.
Two divided line-segments
The values are drawn as two short ruled segments, each cut at an interior point and closed by a curved bracket. The figure carries the numerical pairs of the table into proportional lengths.