Collar

Almost every collar, regardless of style — flat collar, standing band collar, the two-piece shirt collar with its separate stand — is built the same basic way underneath: two identical pieces (an upper and under collar) sewn right sides together around the outer edge, turned right-side out, and pressed so a clean edge shows. What actually distinguishes one collar style from another is the shape of that pattern piece and how much it curves relative to the neckline it's attached to — a flat collar's inner edge curves almost as much as the neckline itself, so it lies flat against the shoulders; a standing collar's inner edge is nearly straight, which is what makes it stand up around the neck instead of lying down.

Interfacing is what actually gives a collar its crispness, and it belongs on at least one of the two layers — usually the upper collar, sometimes both for a firmer result. Skip it and even a well-drafted collar will look soft and a little sad rather than crisp.

The detail that separates a clean collar from an amateur one, in my experience: understitching or grading the seam allowance before turning it right-side out, so the seam rolls slightly to the underside and stays hidden rather than peeking out along the finished edge.