A guillotine cutter takes a thin slice off the head of the cigar. There are single-blade and double-blade versions. A double-blade cutter is gentler on the cap, as long as the blades meet properly. The goal is a clean, round opening that leaves most of the cap in place, so the wrapper does not come loose.
If you cut too deep, the wrapper can unravel. If you cut too little, the draw stays tight. Use the shoulder of the cap, where it curves, as your guide rather than a fixed measurement in millimeters.
The guillotine is the standard tool for the rounded heads of parejo cigars. Pointed figurados sometimes work better with a smaller cut or a v cut. Shop: cutters.