@dansup This sounds like Knuth designing TeX to use a 'scaled point' as his base linear unit with all measurements being integer multiples of 1 sp. 65536 sp = 1 pt. But 72.27 pt = 1 in = 25.4 mm.

So 1 sp = (1000000*25.4/(72.27*65536)) nm = 5.362851 nm and all layout is done in those units.