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I've recently noticed that use of angle brackets for writing tuples, e.g. $\langle x, y \rangle$ instead of the usual round brackets in a few books I've been reading — Lawvere's Sets for Mathematics, Mac Lane's Categories for the Working Mathematician, Forster's Logic, induction and sets, for example. I've also seen occasional use of it in Hartshorne's Algebraic Geometry, but there round brackets seem predominant. Is there some subtle distinction between the two notations I've missed, and what might the reasons for not using round brackets be? Is this practice peculiar to a particular tradition in mathematics (say, foundations)?

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    I'm also curious, since I've encountered abstract algebra texts using angled brackets, e.g., to denote groups : e.g. $\langle \mathbb Z, + \rangle$.2011-06-19
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    In Strooker's *Introduction to categories, homological algebra, and sheaf cohomology*, he uses $\textopencorner x,y\textcorner$ for tuples, but I haven't seen that anywhere else (edit: the corners doesn't seem to be working, they are on page 9 [here](http://ece.uprm.edu/~caceros/latex/specialcharacter.pdf)).2011-06-19
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    It may just be a matter of parenthesis being overloaded.2011-06-19
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    @Arturo: Simple enough!2011-06-19
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    @amWhy: Formally speaking, a structure is a tuple...2011-11-10
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    There was a similar discussion in the comments to [this question](http://math.stackexchange.com/questions/77117/show-that-this-countable-collection-is-a-basis-for-mathbb-r2) - with the connection to notation for intervals and ordered pairs.2011-11-10
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    I've come across angle brackets around tuples to describe a Markov Reward Process, see slide 10 in these [lecture notes](http://www0.cs.ucl.ac.uk/staff/d.silver/web/Teaching_files/MDP.pdf)2017-08-09

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