It is funny, but when I was younger ear flaps were worn for warmth, and those knitted hats usually had a brim as well. I still think the ear flaps are good for warmth, though they do nothing for the appearance.
The brim, though, earns its place entirely.
What a brim actually does
More than decoration.
It doubles the fabric where it matters. A folded brim gives two layers around the forehead and ears – the part of the head that gets cold first.
It grips. A ribbed brim pulls in and holds the hat on, which a plain edge does not. This is why nearly every hat that stays put has ribbing at the bottom.
It adjusts. A generous brim can be folded once or twice, which means one hat fits a range of head sizes and a growing child.
It hides an untidy cast-on edge. Not the main reason, but a welcome one.
Construction
Most brimmed hats are worked bottom up: ribbing for the brim, then the body in whatever stitch you like, then crown decreases.
Knit the brim to twice the depth you want visible, since it folds. This is the most commonly forgotten point – a brim knitted to the finished depth ends up half that once folded.
Work the brim on needles a size smaller than the body. It makes the ribbing grip properly and the transition is invisible.
Fit
A hat should measure an inch or two less than the head, so it grips. Knitted fabric relaxes with wear, and a hat that fits exactly when finished will be sliding down by the end of the season.
Rough guide: baby 14 to 17 inches, child 18 to 20, adult 21 to 23.
For the crown, decrease evenly across several rounds rather than all at once, or the top will pucker.
Ear flaps
Usually worked first, then joined by casting on stitches between them for the front and back.
Make them deep enough to actually cover the ears – shallow flaps look right and do nothing.
For a young child, skip any tie under the chin and knit the hat slightly snugger instead. Cords near a small neck are worth avoiding.
Yarn
Wool or a wool blend has the springiness that makes ribbing grip. Pure acrylic ribbing goes slack over time.
Soft matters – a hat sits against the forehead. And machine washable, because hats are worn constantly and lost regularly.
