Why does an applique file have three colour changes for one fabric?

Each colour change is a stop, not a real colour. The machine pauses so you can lay the fabric (after the placement run) and, if you trim in the hoop, cut it back (after the tack-down). You can sew all three in one thread.

Does the tack-down have to be inside the placement line?

Yes. The placement run marks the edge of the fabric piece; the tack-down must land on fabric, so it sits about 1.2 mm inside. A tack-down exactly on the edge catches only half the fabric and pulls the raw edge up.

How wide should the satin be?

3 to 5 mm on most machines. 4 mm is the common default; knits and fraying fabrics get 4.5 to 5 mm because the fabric pulls in more and the raw edge needs more cover.

Can I skip the underlay under the satin?

You can, but the satin then sinks into the fabric and the edge looks ragged. A light zigzag underlay lifts the satin and holds the fabric edge down before the dense columns arrive.

Placement, tack-down, satin: how an applique embroidery file is built

Every applique design, whether you buy it from a design shop or generate it, is the same three stitch objects in the same order. Once you can see them in a viewer you can judge any applique file in a minute: is the placement line clean, is the tack-down inside it, does the satin cover the edge with margin to spare.

The allowance is the gap between the placement line and the cut line. Too small and the raw edge peeks out from under the satin; too big and fabric shows past it.

Stop 1: the placement run

The placement run is a single running stitch that traces the outline of the fabric piece. It does one job: show you where the fabric goes. The machine sews it on the bare garment, stops, and waits.

Settings that matter:

Stitch length 2.5 mm. Long enough to see, short enough to follow curves. Shorter stitches perforate the garment and leave a visible line if the fabric shifts.
Exactly on the outline. The placement run is the reference for everything else. The cut file is offset from it, the tack-down is inset from it, and the satin is centred on it.
Tie-in and tie-off. Three tiny lock stitches at the start and end, otherwise the first stitches pull out when the machine jumps away.

When you look at a purchased design in the file viewer, block 1 should be a thin closed loop with a few hundred stitches at most. If block 1 is a fill, the design is not applique, or the digitiser used a "placement fill" that you must ignore.

Stop 2: the tack-down

After you lay the fabric inside the placement line, the tack-down fixes it. It is the stitch that decides whether the piece stays where you put it during the satin, which is the heaviest, most distorting part of the design.

Inset 1.2 mm from the outline. The tack-down must land on the fabric, not on the raw edge. Too close to the edge and it lifts the fibres and pulls the edge up into the satin; too far in and it shows as a second line inside the satin when the fabric pulls.
Running stitch, 2.5 mm, or a light zigzag on stretchy fabrics. Blanket or E-stitch tack-downs are decorative and belong to raw-edge applique where there is no satin.
Same path direction as the placement run so the fabric is pushed the same way both times.

If you trim in the hoop rather than pre-cutting, this is the stop where you cut: close to the tack-down, leaving about 1 mm, without nicking the base fabric. Pre-cutting with a cut file skips this step, which is the whole point of the cut file.

Stop 3: the satin border

The satin is a dense zigzag that covers the raw edge and gives the applique its finished look. It is the layer where geometry matters.

Width3–5 mm (4 mm default, 5 mm on knits)

Column spacing0.4 mm

Underlayzigzag, one column per 2 mm, 0.4 mm inside each rail

Bite onto fabricat least 0.8 mm

Pull compensationwoven 0.25 · knit 0.40 · felt 0.15 mm on the outer rail

Two things have to be true at every point along the edge:

The inner end of each satin column sits far enough onto the fabric. We call that depth d₁, and it must be at least 0.8 mm. If the fabric edge drifts inward (a small cut, a slightly off placement) and d₁ was marginal, the raw edge peeks out under the satin.
The fabric does not extend past the outer rail. If the piece is cut too large or placed too far out, fabric shows past the satin and frays.

Both conditions depend on where you cut the fabric relative to the placement line, which is why the cut allowance is a number, not a feeling.

Corners and tips

On a straight edge every column is the same length. At a sharp convex corner the columns fan around the tip and their inner ends bunch up. The engine rotates the columns at sharp corners towards the corner bisector so the inner end stays deep in the fabric, and deepens them until the satin bites at least 0.7 of its half-width. On tight inner curves every other column is shortened by 35 percent so the inner ends do not pile up. These are the two places where cheap auto-digitising falls apart, and the two places the stitch-out prediction inspects most closely.

Underlay

A zigzag underlay is sewn in the same colour as the satin, one column every 2 mm, pulled 0.4 mm in from each rail. It flattens the fabric edge and gives the satin something to sit on. It adds about 20 percent to the stitch count of the border and no extra stop.

Pull compensation

Satin pulls the fabric together across its width. On woven cotton about 0.25 mm, on knits 0.40 mm, on felt 0.15 mm (Hatch ships with 0.20 mm and users raise it to 0.40 when gaps show; OurDigitizer's guidance is 0.3 to 0.5 mm per edge). The outer rail is pushed outward by that amount in the file so the stitched width lands where it was designed. Compensation is never perfect, so the prediction assumes 30 percent of the pull still shows. Read the pull compensation guide for how it interacts with the allowance.

The order of stops in a multi-piece design

A design with three fabric pieces can be sewn two ways:

Layer order: all placements, stop, all tack-downs, stop, all satins. Three stops in total, and you lay all the fabric at once. Faster on the machine, and what most commercial designs do.
Piece order: placement, tack-down, satin for piece one, then piece two. Nine stops. Necessary when pieces overlap and a later piece must cover the raw edge of an earlier one.

The studio offers both. Layer order is the default because most single-colour applique has non-overlapping pieces.

Reading a design you did not make

Open any applique file in the viewer and step through the colour blocks:

Block 1 should be a closed running outline with 100 to 600 stitches. That is the placement run and the shape of the fabric.
Block 2 should be the same outline slightly smaller, or a light zigzag over it. That is the tack-down.
Block 3 should be dense, the same outline, several thousand stitches. That is the satin. Check its width against the design size: under 3 mm on a 100 mm design is thin.
Anything after that is detail embroidery on top of the fabric.

If a design skips block 2, it is a "trim in the hoop" design where the satin doubles as tack-down; those are harder to stitch cleanly on knits. If the satin is not centred on the placement line, the digitiser offset it deliberately, and the cut file tool will show you exactly where the fabric edge was meant to be.

Try it

Generate all three layers from your own shape

Upload an SVG or PNG; the placement run, tack-down and satin are built with the settings above, and the cut file is offset from the same outline.

Open

Sources

Ink/Stitch applique tutorial and satin column documentation, inkstitch.org (open source; our satin geometry was compared against it on ten shapes, inner-rail depth within 0.2 mm).
Hatch Embroidery pull-compensation defaults and Wilcom EmbroideryStudio fabric settings, from their online help.
OurDigitizer digitising guides on satin pull compensation per edge.
Brother PE-Design 11 manual, ScanNCut export dialog (cutting allowance presets 0.0, 1.0 and 2.0 mm).