What is a 'guideline' (guide) in haircutting?

A guideline (or guide) is the section of hair cut first that establishes the length and shape; subsequent sections are combed to and matched against this guide so the cut is even and consistent. Guides can be stationary (all hair brought to one fixed line) or traveling/mobile (the guide moves as you work, common in layered cuts). Working from a guideline keeps a haircut controlled and balanced. Understanding what a guideline is and how it controls a cut is core haircutting theory.