Satin density: the number we measured instead of quoting
Digitising manuals give satin densities that differ by a factor of two because they count differently. We measured commercial designs and set the lettering tool to match.
2 min read · Published September 26, 2026
Ask three digitisers for the right satin density and you get 0.4 mm, 0.2 mm and 4.5 lines per millimetre. They all mean the same thing, and it took us a bad batch of test letters to see why.
Two ways to count
A satin column is a zigzag. The needle goes down on the left rail, then the right, then the left. You can count the spacing between consecutive penetrations on the same rail, or the distance the column advances between any two penetrations, which is half of that.
Wilcom and Hatch quote the first: "satin spacing 0.42 mm" is between same-rail penetrations. PE-Design quotes "4.5 lines/mm", and a line is one zigzag leg, so 4.5 legs per millimetre is a 0.44 mm same-rail spacing. Embird's "density 0.2" is the advance per leg: the same thing again. Every manual is describing 0.40 to 0.45 mm between penetrations on a rail.
Our first lettering build took the 0.2 figure as same-rail spacing. The letters were beautiful on screen and twice as dense as anything a machine wants; the pre-flight check on our own output lit up red, which was at least a good sign for the check.
What the corpus said
Rather than pick a manual, we measured. Across the commercial designs in our test corpus (three shops, a few hundred satin columns), the median same-rail spacing was 0.40 to 0.41 mm, and the spread was narrow: professionals agree with each other even when their manuals phrase it differently. Hobby and auto-digitised files ranged from 0.20 to 0.61 mm. The 0.2 files are the ones that break thread; the 0.6 files show fabric through the column.
So the lettering tool's satin is 0.40 mm same-rail, which the engine implements as a 0.20 mm walk step along the column, alternating rails. On 40-weight thread that puts each leg next to the last with no gap and no stacking. The pre-flight check's density threshold, six penetrations per square millimetre, is set so a single 0.4 mm satin (about 2.5 per mm²) never trips it and two stacked ones do.
Why it matters beyond letters
Resizing multiplies density: scale a 0.4 mm satin to 70 % without correction and the same stitches sit at 0.28 mm, most of the way to breaking thread. The resize tool re-samples the zigzag to keep the original pitch, which is only possible because we know what the pitch should be. The density guide has the numbers for fill as well; the lettering guide has what they mean for text.