The undercollar provides structure and shape to the visible top collar of a jacket or coat. It is typically cut slightly smaller than the upper collar and often on the bias, which forces the seam line to roll underneath and remain invisible from the outside. In bespoke tailoring, it is frequently made of specialized materials like melton wool or stabilized with pad stitching.
Heard on the job
Pad stitch the undercollar to the canvas first, making sure the roll line is crisp before attaching the top fabric.