Two designs can look identical on screen and sew completely differently, and embroidery underlay and density are usually the reason. Underlay is hidden under the finished stitching, and density is just a number in the object properties, yet together they decide whether a logo sits flat and crisp or turns stiff, puckered or patchy. This guide covers the main underlay types, how density is measured, and how to choose both for different fabrics.
What embroidery underlay does
Underlay stitches go down before the visible top stitching of each object. They do several jobs at once:
- Stabilize the fabric by tacking it to the stabilizer so it cannot shift or stretch under the top stitches.
- Lift the top stitches slightly so they sit on the surface instead of sinking into knits or pile.
- Hold down nap on fleece, towels and velvet so fibers do not poke through.
- Improve edges by giving satin columns a firm rail to wrap around.
Underlay also works with pull compensation: a well-anchored fabric moves less, so shapes hold their size. Our guide to pull compensation in embroidery covers that side of the balance.
Types of embroidery underlay
| Underlay type | How it sews | Typical use |
|---|---|---|
| Center run (center walk) | A single running stitch down the middle of a column | Narrow satins, roughly under 2 mm, and small lettering |
| Edge run (contour) | Running stitches just inside the edges of a shape | Medium satins, and around fills to define crisp edges |
| Zigzag | An open zigzag across the column | Wider satins, often combined with edge run, to add loft |
| Double zigzag | Two zigzag passes crossing each other | Wide satins, soft fabrics and designs that need extra support |
| Tatami (fill) underlay | An open fill, usually at an angle to the top fill | Fill areas, towels, fleece and knits |
Combinations are common. A wide satin on a polo might get an edge run followed by a zigzag, and a large fill might get an edge run plus a tatami underlay running at roughly 90 degrees to the top stitches. Tiny details may get no underlay at all, because extra stitches in a very small area only add bulk.
What embroidery density means
Embroidery density describes how close together the top stitches are. It is usually set as the spacing between rows in millimeters, so a smaller number means denser stitching. Some programs express it differently, for example in points or lines per unit, so check your software's units before copying values.
- Standard fill: around 0.4 mm spacing on stable fabric.
- Satin columns: often similar or slightly denser, with narrow columns needing a little more space so they do not pile up at curves.
- Lighter density: useful on knits, on large fills, and where underlay already provides coverage.
Too dense, and the design becomes stiff, puckers the fabric, breaks thread and needles, and adds stitch count. Too light, and the fabric shows through. Density is a balance, not a quality score.
How underlay and density interact
Underlay adds thread under the top layer, so it affects how dense the top needs to be. Strong underlay on a textured polo can let you open the top density slightly and still get full coverage, keeping the design flexible. Skimping on underlay usually leads to digitizers compensating with heavier top stitching, which creates stiffness without fixing the real problem.
Some specialties change the rules. For 3D puff, the foam areas get little or no underlay so the foam can rise, and the satin on top is set so it covers the foam and cuts it cleanly at the edges. See 3D puff digitizing for details. On towels, a light knockdown fill flattens the loops before the design sews, as explained on our towel digitizing page.
Underlay and density by fabric
| Fabric | Underlay | Density | Notes |
|---|---|---|---|
| Woven twill, denim, canvas | Edge run and zigzag on satins; tatami under fills | Standard | Most forgiving group |
| Piqué polos | Firm underlay: edge run plus zigzag, tatami under fills | Standard to slightly lighter | Texture needs support so stitches do not sink |
| Jersey T-shirts and performance knits | Tatami and edge run to lock the fabric | Lighter | Cut-away stabilizer; avoid heavy fills |
| Fleece and sweatshirts | Tatami underlay to hold the nap | Standard | Topping helps on high pile |
| Towels and terry | Knockdown fill plus underlay | Standard, satins kept full | Water-soluble topping keeps loops down |
| Structured caps | Center or edge run plus zigzag | Standard | Sew order from center outward |
| Beanies and knit hats | Substantial underlay | Lighter | Knit stretches in the hoop |
| Leather and vinyl | Minimal | Lighter | Every penetration perforates the material |
Treat the table as a starting point. Thread weight, needle size, stabilizer and hooping all change the result, so a test sew-out on the real fabric is always the final check.
Common underlay and density problems
- Fabric showing through fills: density too light for the fabric, or missing tatami underlay.
- Design feels like a stiff patch: density too high, too much underlay, or overlapping objects stacked on each other.
- Puckering: dense stitching on unstable fabric, often with weak stabilizer or loose hooping.
- Ragged satin edges on knits: no edge run, so the column edges sink into the knit.
- Thread and needle breaks: very dense areas, especially where layers overlap.
Checking density in a file you already have
Compare the stitch count with the design size. A solid fill runs roughly 1,800 stitches per square inch, and our stitch estimator gives a quick benchmark from size and coverage. A count far above the estimate suggests overlapping objects or heavy density. Note that resizing a machine file like DST keeps the same stitch count, so shrinking it makes it denser and enlarging it makes it lighter.
StitchQuill chooses underlay and density for the fabric you name in your order, and every file comes with a PDF worksheet showing stitch count and size. Start an order