Underlay
Underlay is a hidden base layer of stitches sewn first, under satin or fill stitching, that flattens the fabric and gives the top stitches something firm to sit on.
Underlay is a set of stitches the machine sews before the visible satin or fill stitching. You rarely see it in the finished design. It is part of the stitch file, put there when the design was digitized, not something the machine adds on its own.
It matters because thread on its own is a poor base. A single layer of fill stitching tends to sink into the fabric, look sparse and let the fabric bulge between rows. Underlay flattens the fabric and gives the top stitches rails to sit on. It also helps tack the fabric and stabilizer together, and on satin it keeps the edges neat.
There are three common types. A center run is a single line of stitches down the middle of a satin column and back. An edge run (Ink/Stitch calls it contour underlay) goes up one side and down the other, set slightly in from the edges so it does not peek out around the column. Zigzag underlay is a looser, lower-density satin sewn over the width. Thin columns may need only a center run, while wide columns or tricky fabrics often get all three. Fill underlay is usually fill stitching at 90 degrees to the top fill, with rows spaced much wider. Software names differ, so look for center run, edge run and zigzag or similar wording.
You adjust underlay in digitizing software, where you can switch it on or off and change its type, spacing and inset. Stretchy knits and towels usually do better with a lighter top fill and a good underlay than with a denser top fill. In a finished stitch file the underlay is just stitches in the sequence, so changing it means going back to the editable design.
Common mistakes: leaving underlay off on a knit or towel, so the satin sinks in; stacking heavy underlay under an already dense top layer, which adds stitches and pull and invites puckering; and not insetting the underlay enough, so it shows at the edges.
Example: A bold satin column about 0.4 in (10 mm) wide on a T-shirt knit could carry all three types: a center run, an edge run inset from each side and a zigzag, with the top satin spaced about 0.016 in (0.4 mm) apart for 40-weight thread. A thin satin outline about 0.12 in (3 mm) wide on stable woven cotton may need only a center run. These widths are illustrative starting points, not rules.
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Frequently asked questions
Does every embroidery design need underlay?
Not always, but most filled and satin areas benefit from it. Ink/Stitch turns fill underlay on by default and describes switching it off as a rare case, such as a fill with very wide row spacing.
Why does my satin stitch look thin or sink into the fabric?
Missing or too-light underlay is a common cause, especially on knits and towels, which tend to need a good underlay. Check your stabilizer and hooping too, since fabric that shifts or stretches can cause the same look.
Can I add underlay to a design I downloaded?
Usually not easily. A finished stitch file is a list of stitches, and your machine cannot generate underlay from it. Changing underlay normally means opening the editable version of the design in digitizing software, or having it digitized again.