Trapping.

Extending one colour a little under or over its neighbour, so that when screens or passes land a fraction off, the garment does not show through between them. It is the colour-to-colour counterpart of choke, which pulls the white underbase in under the top colours. Thomas Trimingham in Screen Printing Magazine sizes it from a test print: double the largest gap you see, so a 1/64 inch gap calls for a 1/32 inch trap, and specialty items such as mugs and balloons can need much more. The same article warns that too much trap brings bleeding, lost detail and colour shift where the inks stack. The point figures printed beside those fractions are twice their true values, so work from the fractions. Trap is a separation setting, not something a seller adds by hand to a file sent to a print-on-demand platform, but a design built from colours that only touch at hairline edges, with no outline and no gap, is the one that needs it most. Skip it and a halo of shirt colour appears along every edge on a press that has drifted by a thread's width.