I don't think it's that common. At least, I don't recall seeing it ever. Maybe it's used in non-English mathematics?
Wikipedia mentions it's also used in electrolysis so maybe this new one is related to that somehow?
So for "not a right angle" I'd have expected a "right angle" symbol with a slash through it.
And some commenters said they used it to mark proof by contradiction, but why is there a need to mark it when you are showing it via proof? A canonical example of proof by contradiction is proving sqrt(2) is not rational. Never have I seen it marked with that symbol. Where would you even mark it? At the beginning with the assumption? Or at the end like QED?
"By way of contradiction suppose P, then ..., thus ~P ↯. Therefore ..."
[0] https://en.wikipedia.org/wiki/Contradiction#Symbolic_represe...
https://emojipedia.org/person-in-suit-levitating/
(Edit: Apparently HN automatically removes emoji.)
https://en.wikipedia.org/wiki/Miscellaneous_Technical
Aside from the table describing each symbol, if you scroll to the bottom of the page, it links out to full articles related to each. For a full list see...
This symbol doesn't have that box, so I don't think it's a right angle.
[0] https://en.wikipedia.org/wiki/Right_angle#/media/File:Right_...
Edit: This merely adds to the confusion, since the name of the glyph contains the words "right angle."
¯\_(ツ)_/¯
> In Unicode, the symbol for a right angle is U+221F ∟ RIGHT ANGLE (HTML ∟ · ∟). It should not be confused with the similarly shaped symbol U+231E ⌞ BOTTOM LEFT CORNER (HTML ⌞ · ⌞, ⌞). Related symbols are U+22BE ⊾ RIGHT ANGLE WITH ARC (HTML ⊾ · ⊾), U+299C ⦜ RIGHT ANGLE VARIANT WITH SQUARE (HTML ⦜ · ⦜), and U+299D ⦝ MEASURED RIGHT ANGLE WITH DOT (HTML ⦝ · ⦝).[5]
> In diagrams, the fact that an angle is a right angle is usually expressed by adding a small right angle that forms a square with the angle in the diagram, as seen in the diagram of a right triangle (in British English, a right-angled triangle) to the right. The symbol for a measured angle, an arc, with a dot, is used in some European countries, including German-speaking countries and Poland, as an alternative symbol for a right angle.[6]
The article notes that sans a given meaning the glyph was given a “descriptive name”.
So you’re not wrong? :-P
Any examples?
Yes, we indeed used and afaict still use the thunderbolt for contradiction in my German university.
However "perpendicular" and "bottom/falsum" are two different Unicode codepoints with very similar glyphs.
But maybe "commonly used" was maybe the wrong term. More appropriately: "sometimes" or "by some".
http://mirrors.dotsrc.org/ctan/info/symbols/comprehensive/sy...
There are lots of examples of the lightning bolt in there. In fact, under ulsy Contradiction symbols, there are four variants.
I also noticed the exact symbol being discussed is listed under "Angles".
(A) It's in the math section, (B) it's with angles, (C) the thunderbolt ↯ is commonly used for "not" or more specifically for dis-proof in this area and
(D) at least by my 30 s internet search on a mobile phone I couldn't find any other "no-angle" or "no-right-angle" symbol.
Someone could argue that usually you use a simple strike through as like as in ≠ (unequal), ∉ (not-element-of) or ∅ (empty set) but I would say it was chosen to avoid confusion in this case. The angle itself (without the "no/not") consists of only to orthogonal lines so it would be kinda complicated to "strike it though" in any direction without ambiguity that would resemble a triangle, a fork or whatnot.
■