You'll need: FontCreator (Windows and macOS) and a variable font project with at least two masters.

Variable font compatibility is the single thing that decides whether your masters interpolate into clean intermediate weights and widths — or fall apart the moment a user drags the weight slider. In FontCreator, the font editor flags every incompatible glyph for you, but the red icons and their cryptic letters only help if you know what each one means. This guide decodes FontCreator's master-compatibility indicators (O, M, F, N, D, S, W, T, A) and shows how to fix interpolation, on Windows and macOS alike.

What "compatible" means for interpolation

A variable font builds every in-between style by interpolating between masters. For that math to work, the masters must describe the same shape topology — point for point, contour for contour, component for component. Interpolation requires at least two masters with compatible outlines.

Concretely, each master of a glyph must agree on:

• the number of contours and components,
• the number and type of on-curve and off-curve nodes on each contour,
• the direction each contour runs,
• the start point of each contour,
• the order contours and components appear in,
• the transform applied to each component, and
• the anchors attached to the glyph.

When two masters disagree on any of these, the intermediate instances either snap to wrong shapes or refuse to build.

Note: Overlaps are explicitly allowed in variable fonts. You do not need to remove overlap to be compatible — and you generally shouldn't, because removing overlap per master can change point counts between masters and break interpolation. Keep overlapping contours in a variable font; resolve them only on static export.

Where compatibility is shown

Open the Masters and Layers panel. It lists all masters of the font, and under each master row the layers attached to it. Next to any master whose current glyph is incompatible, FontCreator shows a red icon followed by one or more letters identifying the kind of issue.

Decoding the indicator letters

Each red letter points at exactly one class of mismatch. Read them as a checklist — fix the structural ones first, because they often make the smaller ones disappear.

• O — Outlines. A general outline incompatibility between masters. Treat it as a prompt to inspect the glyph's contours master by master.
• M — Mismatch in number of contours or components. One master has more (or fewer) contours or components than another. This is the most fundamental break: there is nothing to interpolate against. Add the missing contour/component, or remove the extra one, so every master has the same count.
• F — Mixed quadratic and cubic contours. Within the same glyph, one contour is quadratic (TrueType) and another is cubic (PostScript). Convert the glyph so all its contours use one curve type across all masters.
• N — Mismatch in on-curve and off-curve nodes. The contours line up in count, but their points don't: a node that is on-curve in one master is off-curve in another, or one master simply has more points on a contour. Insert or convert points so each corresponding contour has the same node sequence.
• D — Different contour directions. A contour runs clockwise in one master and counter-clockwise in another. Reverse the direction on the offending master so all masters agree.
• S — Mismatched start points. Contours match in points and direction, but the starting node sits in a different place. Set the start point to the same node in every master.
• W — Swapped contours or components. The contours/components are all present but listed in a different order across masters, so master 1's contour 2 is interpolating against master 2's contour 3. Reorder them so the sequence matches.
• T — Component transform mismatch. A component is scaled, rotated, or skewed differently between masters. Bring the transforms into line, or decompose the component if it can't be reconciled.
• A — Anchor mismatch. The glyph carries different anchors (or anchors in a different order) across masters. Anchors interpolate too, so add, remove, or align them so every master has the same set.

Tip: Letters often cascade. Fix M (counts) first; that frequently clears N, S, W, and D that were really just symptoms of contours not matching up in the first place.

Bulk fix vs. per-glyph fixes

FontCreator can attempt to make masters compatible in bulk, but the two approaches serve different moments in a project.

1. Select the glyphs you want to reconcile in the Font panel (or work inside a single glyph in the Glyph Edit window).
2. In the Masters and Layers panel, click Make Compatible on the panel toolbar to let the font editor align contour order, directions, and start points automatically across masters. (It's a split button — its dropdown also holds Fix Kinks, covered below.)
3. Re-check the Masters and Layers panel: any glyph still showing a red icon needs a manual pass.

For per-glyph repair, open the glyph and work directly on the contours of the master that is flagged:

• Align the start point — to clear S. When masters disagree on the start node, Make Compatible moves it to match; the compatibility overlay also marks the start node it recommends, so you can confirm the result master by master.
• Change Direction — to clear D.
• Reorder contours/components — to clear W.
• Insert points / convert on-curve↔off-curve — to clear N.
• Add or remove a contour/component — to clear M.
• Align component transforms (or decompose) — to clear T.
• Add, remove, or reorder anchors — to clear A.

Watch out: Don't blindly auto-fix. Bulk reconciliation guesses at correspondences; on a glyph where masters genuinely differ in design (an alternate terminal, a different counter shape), an automatic match can pair the wrong contours and produce ugly interpolation that still passes the compatibility check. Spot-check the result by previewing intermediate locations.

Kink detection — a separate quality check

Compatibility is about whether a glyph interpolates. Kinks are about whether it interpolates smoothly. A "kink" is a point where a curve that should stay smooth develops a visible corner at intermediate locations, because the handles on either side don't move in step between masters. This is a distinct quality check from the O/M/F/N/D/S/W/T/A indicators — a glyph can be fully compatible and still kink.

FontCreator does flag kinks for you. The Font Validation wizard (Font → Validate…) includes an Interpolation kinks check, and the Masters and Layers panel toolbar offers a Fix Kinks command (in the Make Compatible split button's dropdown) that nudges the handles back into line. After your masters are compatible, run the kinks check or preview interpolated locations and watch smooth curves for corners appearing partway along the axis; then use Fix Kinks, or adjust handle positions on the masters by hand, so the curve stays smooth across the whole range.

Frequently asked questions

Why won't my variable font export, even though each master looks fine on its own? Interpolation requires at least two masters with compatible outlines. If any glyph shows a red icon in the Masters and Layers panel, the masters disagree on point/contour structure. Each master looking correct in isolation isn't enough — they must share the same topology. Clear every indicator before exporting.

Do I need to remove overlaps to make masters compatible? No. Overlaps are explicitly allowed in variable fonts, and removing overlap per master can change point counts between masters and break interpolation. Keep overlaps in the variable font; resolve them only when you cut static styles on export.

What does the letter next to a master mean? It identifies the kind of incompatibility: O outlines, M contour/component count, F mixed quadratic and cubic contours, N on-curve/off-curve node mismatch, D contour direction, S start point, W swapped contours/components, T component transform, A anchor mismatch. Fix the count mismatch (M) first; the others often resolve once contours line up.

Should I trust the automatic Make Compatible result? Use it as a fast first pass, then verify. Automatic matching can pair the wrong contours on glyphs whose masters differ by design, producing a "compatible" glyph that interpolates badly. Always preview intermediate locations after a bulk fix.

What to read next

• How to Make a Variable Font — define axes and masters, preview interpolation, and export named instances.
• Named Instances & Static Font Export — turn your now-compatible design space into shippable static styles.
• Export a Font: TTF, OTF, WOFF & WOFF2 in FontCreator — outline format, color, hinting and variable-font export settings.