MEN'S FLANNEL SHIRTS
Flannel is a finish rather than a pattern. The cotton is brushed after weaving, which raises the surface and traps air.
Why it's warmer
That brushed surface makes a flannel shirt noticeably warmer than a standard cotton shirt of the same weight, and softer against the skin. It sits in the same slot as a light jumper, with the advantage of a collar.
Patterns and fit
Check is the pattern most associated with the fabric, from larger blocks to finer tartan-derived designs. Regular fit gives room to wear it open over a t-shirt. Wash cool. See overshirts for something heavier and casual shirts for lighter weaves.