All United States coins have three sides. Collectors pay most of their attention to the obverse (the front or side), and the reverse (the back or side). Those are where the of a coin - its design, date, denomination, mintmark, and so forth - are found. But the edge also includes interesting information, and in some cases, what's on the edge can make a huge difference in desirability.

An or is a steel disc about five to 10 millimeters thick, 100 mm in diameter, with a circular hole cut in the center. For plain edge coins, or those with incuse ornaments, this hole is the exact diameter of the finished coin. If a coin has a reeded edge, then the hole's diameter is that of a finished coin less the depth of the reeds. In this way the completed coin has its correct diameter when measured over the reeds. A similar approach is used when a design has raised ornaments on the edge.

There are three basic types of edge dies, or collars, that were used by the United States Mint: open, close, and segmented. All three collar types were supported in the press by heavy springs. This allowed the collar to move downward if it were accidentally struck by the hammer die or when a planchet did not seat fully in the collar.

An open collar was used for most U.S. coin denominations from 1792 through part of 1828. This was followed for smaller diameter coins with a close collar in 1829, then larger coins, and finally, a segmented collar for eagles and double eagles, beginning with the Saint-Gaudens designs of 1907-1933.

To read the complete article, see:
Edge Collars - Coining's Third Dimension(https://coinweek.com/edge-collars-coinings-third-dimension/)