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I noticed, that while using unicode-math, both \hbar and \hslash produce the same output, i.e., always an \hslash (actually, the package defines \def\hbar{\hslash} on line 2159). However this output is in contrast with many publications, e.g., Wikipedia or scienceworld.wolfram.com. In Wolfram Mathematica, text uses \hbar, while code uses \hslash.

The description in the unicode-math package's documentation

These are symbols with multiple names.

hints towards a misunderstanding by the package authors, because there IS a difference.

So, is there any rule, when to use which symbol. Is this only a matter of taste?

closed as primarily opinion-based by Werner, Stefan Pinnow, egreg, Kurt, user31729 Jun 10 '16 at 18:43

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    As a physicist, I'd say it's a sheer matter of taste. It may of course depend on whether the font has both or not. – campa Jun 10 '16 at 14:36
  • What the difference in meaning should be? – egreg Jun 10 '16 at 15:22
  • @egreg: As a physicist: I don't know there's a difference in the meaning of \hslash and \hbar typesetting such that it would result in a significant change of meaning what those symbols represent. – user31729 Jun 10 '16 at 17:57
  • From your comments, David's answer and the hold of the question, I conclude that this is a matter of taste. Thanks! – Stefan Jun 13 '16 at 5:34
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unicode-math maps TeX names to Unicode and Unicode decided that these were just stylistic variants so there is only one slot, U+210F.

That means there is only one glyph in many Unicode fonts as well, although of course a font can offer multiple variant glyphs for the same Unicode slot.

the mappings from the ISO entity sets as used in MathML and HTML have the same issue, ISOAMSO defined

planck: /hbar /hslash - Planck's over 2pi
plankv: /hslash - variant Planck's over 2pi

and those &plank; and ℏ names are valid in HTML, but they both resolve to U+210F.

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