A while ago I found out that there are two hardly known units available at least in
nc for a Didot point of 0.375 mm as exactly as TeX can represent it in
sp and the corresponding cicero of 12 points. From Q319652:
So 1238/1157 pt/dd or pc/cc is indeed exact and there are actually two units for the “metric Didot point” of 375 µm and its cicero:
- 1nd = ⌊685/642 pt⌋ = 69925sp ≈ 0.375mm
- 1nc = ⌊1370/107 pt⌋ = 839105sp ≈ 4.5mm
However, these units are not mentioned in Knuth's TeX Book and tex.web §458. When have they been added and where are they available?