crossing

《Symplectic Geometry》 · 第 52–53 页 · Sections 3.2–3.3，regular projection 与 Definition 3.3.1 中 crossing 的说明

Let \(L\subset\mathbb R^3\) be a link and let \(\pi:\mathbb R^3\to\mathbb R^2\) be a projection map. […] If \(\pi(L)\) has a finite number of singular points and they are all transverse double points, the projection is said to be regular (see Figure 3.2). […] A diagram is a regular projection of a link that has relative height information added to it at each of the double points. The convention is to make breaks in the line corresponding to the strand that passes underneath. The double points in the projection become crossings in the diagram.