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Underbase choke: how much, and why

PrintSeps guides · updated October 2026

A white underbase that matches the color art edge for edge will show a white halo wherever the press is a hair out of register, which on a shirt is everywhere. The fix is the choke: shrink the underbase inward so the color plates overlap its edge and hide it. This guide gives the numbers and the reasons behind them.

What the choke does

Choking pulls every edge of the underbase plate in by a fixed distance. The color printed on top then extends slightly past the white, and the edge of the print is color meeting shirt, not white meeting shirt. Misregistration of less than the choke distance stays invisible. The price is a thin band around every shape where color prints directly on the garment, slightly duller than the rest; keep the choke as small as your press allows so that band stays thin.

Starting numbers

If you only remember one number, use 1 point. Points are the unit most separators quote; multiply by your film resolution divided by 72 to get pixels (1 pt at 300 dpi is 4.2 px, so round to 4).

What pushes the number up or down

• Press. Automatics hold registration tighter than manuals, so they can take a smaller choke.
• Mesh on the underbase screen. A 110 mesh base deposits a heavier, more spreading layer than a 160 mesh; the thicker deposit effectively grows the edge, so give it more choke.
• Garment. Fleece and heavy cotton move more on the platen than a ringspun tee.
• Art style. Bold vector shapes hide a larger choke easily. Fine line work, small type and photographic edges cannot: thin strokes narrower than twice the choke lose their base entirely and print dull. For those, choke less and register harder, or accept that very thin elements print without base.
• Ink. A high-opacity color ink hides the base edge better than a thin one, so it tolerates a smaller choke.

How to apply it

1. Build the underbase as a grayscale plate first, at final print size and resolution.
2. Apply a minimum filter, contract, or "spread/choke" operation of the chosen radius. Do it on the base plate only; color plates keep their true edges.
3. Soften the result by a fraction of a pixel. A hard-edged choke can leave a visible step on curves after halftoning.
4. Check the thinnest elements. Anything that disappeared from the base is going to print directly on the shirt: decide if that's acceptable or if the choke needs to come down.

Choke vs. trap

Choke shrinks a plate; trap (spread) grows one. In screen printing the convention is to choke the underbase rather than spread every color, because the base is one plate and the colors are many, and because an overgrown color plate would change the art. The exception is a light color butting against a dark one with no base: there, spreading the dark ink slightly under the light one hides the gap.

Highlight white is not choked

The highlight white, printed last on top of the colors, is a separate plate and is usually left unchoked or even spread a touch, since it sits on ink, not shirt, and a hairline of extra white reads as sparkle rather than a halo.