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The PD (Planar Diagram) form for a virtual knot diagram is obtained
by numbering the edges of the diagram in ascending order, beginning at
some point and travelling along the diagram. Virtual crossings are
ignored, and after each non-virtual crossing is encountered, the new
edge is numbered. Then, at each crossing, list the edges, beginning
with the edge incoming on the under strand, and proceeding
counter-clockwise. Crossings are indicated like
X[*i*,*j*,*k*,*l*], where *i*,
*j*, *k*, and *l* are the four edges at that
crossing.

For example, the PD of virtual knot 2.1, seen below, is X[2,4,3,1], X[3,1,4,2].

See also: Planar Diagrams at Dror Bar-Natan's Knot Atlas.