Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
One group of Regular (R) and three groups of Irregular shapes: Irregular Complex (IC), Irregular Simple Curved (ISC) and Irregular Simple Straight (ISS). A group of 16 shapes corresponds to two consecutive rows (labeled a,b) and the group names are depicted in the tree graph on the right. The 8 pairs (a,b) of each group are defined by the numbers on top of the figure (1,2, …8).
