Congruency Notation
In geometry, the notation for equal measure of angles, arcs, line segments, or other elements of a diagram is hatch marks. They are short line segments used in a pattern to indicate relationship, specifically congruency.
Hatch marks for congruency notation are in the style of tally marks or of Roman numerals – with some qualifications. These marks are without serifs and some patterns are not used. For example, the numbers I, II, III, V, and X are used, but IV and VI are not used since a rotation of 180 degrees can make a 4 easily confused with a 6.
Considering the close relationship of geometry to art, hatch marks can probably be traced to hatching in which the pattern of the marks is representative of a unique tone or hue with different patterns indicating different tones. Likewise in geometry, each hatch pattern is representative of a unique value. When hatch marks match, measurement values for the two items are the same.
For example, if two triangles are drawn, the first pair of congruent sides can be marked with a single hatch mark on each. The second pair of congruent sides can be marked with two hatch marks each. The patterns are not alike: one pair uses one mark while the other pair uses two marks (Figure 1). This use of pattern makes it clear which sides are the same length even if the sides cannot be measured. If the sides do not appear to be congruent, as long as hatch marks are present and are the same number of hatch marks then the sides are congruent.
Read more about this topic: Hatch Mark