# What is the term for the marks used to show congruence in geometric figures?

When looking at a given picture to be used in a geometric proof, often times single, double, or triple "slashes" mark off equal line segments or arcs. What is the correct term for these? I've seen hatch marks, hash marks, tick marks, tic marks, slash marks, etc. Wikipedia uses the term hatch marks here, but I'd rather not just take their word for it.

• Cross posted at math.stackexchange.com/q/2304271/35416. Please do not do that.
– MvG
Commented May 31, 2017 at 23:20
• I will tailor better to each audience in the future. I would like both the mathematician's perspective, and the educators's perspective. Commented Jun 1, 2017 at 17:14
• "What is the correct term for these" what makes you think that there is a single ("the") correct term for these? Commented Jun 3, 2017 at 15:53

While I doubt very much that there is an official or canonical name for these markings, the most detailed analysis of the way geometric diagrams marshal semiotic resources is probably in an article by Justin Dimmel and Pat Herbst, "The Semiotic Structure of Geometry Diagrams: How Textbook Diagrams Convey Meaning", published in JRME in 2015. (http://www.jstor.org/stable/10.5951/jresematheduc.46.2.0147). Dimmel & Herbst refer to those marks as "hash marks" (p. 166), and identify them as examples of a larger system of notational devices that they call "geometric diacriticals". These, in turn, are part of a subsystem of "relational attributes", which belong to a larger "attributes system". The relevant passage is:

Relational markings are the symbols that convey geometric properties when they are applied to parts in the geometry diagram. This set of symbols includes the small squares (i.e., quadrants) that indicate right angles, small arcs that indicate congruent angles, hash marks that indicate congruent segments, and sets of arrows that indicate sets of parallel lines. These markings are resources through which the diagram directly communicates geometric properties without any supporting literal or symbolic statements. Thus, an angle marked by a quadrant is a right angle, regardless of whether it is visually perceived as a right angle (see Gal & Linchevski, 2010). We refer to relational markings as geometric diacriticals because they signal geometric properties of objects in diagrams analogously to the way that diacritical markings (e.g., accents, tildes, umlauts) signal phonetic properties of letters in words.

(This is a really good paper, by the way.)