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Where/when did the convention originate of marking pullback (and/or pushout) squares by that little right-angle symbol in the corner?

Pair of pullback squares; from Spivak, Category Theory for the Sciences; fair use for illustrative purposes of the use of this notation in the literature

The earliest instance I’ve been able to find is in Paul Taylor’s diagrams package, from ≤1994, as mentioned in e.g. the changelog notes for v3.81 at http://www.paultaylor.eu/diagrams/ . But it seems more likely that this was to meet the demand for a notation that was already established, rather than being the origin? But looking at various well-known category theory textbooks from before 2000 (Mac Lane Categories for the Working Mathematician; Mac Lane and Moerdijk Sheaves in Geometry and Logic; Borceux Handbook of Categorical Algebra; Johnstone Topos Theory), none of them seem to use it, as far as I can find.

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    $\begingroup$ @ToddTrimble I strongly suspect it has been invented by him. I've seen it in several papers of Peter Freyd, as well as in Cats and Allegators. The latter is full of notation I wish would become standard, like puncture sign for non-commuting diagrams. $\endgroup$ – მამუკა ჯიბლაძე Mar 26 '17 at 19:09
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    $\begingroup$ Indeed, the Freyd-Scedrov book from 1990 has the notation: books.google.com/… $\endgroup$ – Todd Trimble Mar 26 '17 at 19:11
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    $\begingroup$ While we're at it maybe we can standardize this notation too? I've seen that right angle all over the place and pointing in several different directions. $\endgroup$ – Jonathan Beardsley Mar 27 '17 at 4:49
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    $\begingroup$ In Freyd's notation the corner is actually attached to the arrows. He also seems to use a similar cross notation for products, and several other notations for (co)equalizers etc. $\endgroup$ – Dmitri Pavlov Mar 27 '17 at 9:41
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    $\begingroup$ @მამუკაჯიბლაძე: That's one point of view (i.e., a corner is an arrowhead). Another point of view states that a corner is meant to represent the two newly constructed arrows; in the case of a pushout these two arrows give you ⌟. And a third point of view states that a corner is meant to represent the two old arrows, which would give you ⌜ for a pushout. $\endgroup$ – Dmitri Pavlov Mar 29 '17 at 11:20
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A small comment, that does not fit in its proper place. The department of Algebra in the University of Santiago de Compostela has studied categories since the second half of the sixties under the leadership of prof. Eduardo García-Rodeja. I made my studies later and the following notations were standard:

• Cartesian square:

enter image description here

• coCartesian square:

enter image description here

So, I was not surprised to see Paul Taylor's notation. And for me, too, it was the first time I noticed a notation similar to the one it was used here in print. later, the main research areas where closed categories and homology theories, though after all these years, it has diversified a lot.

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    $\begingroup$ Very interesting, thankyou! What was your department then, and who were the people who you saw using this — either in terms of individuals, or in terms of what research area(s) they were in? $\endgroup$ – Peter LeFanu Lumsdaine Mar 27 '17 at 11:13
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    $\begingroup$ I'va added some precisions. For me, personally, it was very useful to have a good background in categories to study cohomology in algebraic geometry. $\endgroup$ – Leo Alonso Mar 27 '17 at 11:30

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