Start with the cloth
A loose or open weave may need a thread that fills the surface without disappearing into the gaps. Fine cloth can become crowded if paired with a heavy thread and dense pattern.
Run a short length through the fabric before choosing. The right weight should pass without excessive drag and sit on the surface without forcing the weave apart.
Consider pattern scale
Thick thread makes large repeats and simple lines easy to see from a distance. Fine thread supports small motifs, delicate borders and projects where the cloth should remain the main visual feature.
If a pattern has both tiny corners and long open runs, use a middle weight or simplify the small details. Changing thread weight mid-design can look accidental unless planned.
Test flexibility and wear
Dense thick stitching can add structure to a patch or bag, but may feel stiff across clothing. Fine thread often keeps a repair more flexible when the garment bends frequently.
Wash or gently handle the swatch as the finished piece will be used. A thread that looks right before movement may need different spacing once the cloth relaxes.