As discussed in this answer https://physics.stackexchange.com/a/77691/667 there are several common conventions for the notation $[q]$ of a physical quantity $q$.

However, I often see people to put the square brackets around the (SI-) unit itself. For example $[\mathrm{kg}]$ (for kilogram). Most time the people using this are over 50 years old and often engineers. So, I got the impression that this convention was common some years ago.

So, what are the origins and reasons for this old convention?

  • $\begingroup$ I don't follow the reason for your comment "though from a modern point of view it doesn't seem to make sense". In particular, what do you mean by "modern" with respect to not making sense. As for people who are over 50, including me (my age is in open interval $69.551 < \textrm{my age} < 70$, I have always thought the $[]$ bracket notation as used in dimensional analysis was useful (not sure though that "making sense or not making sense" is applicable). $\endgroup$ – K7PEH Apr 20 '17 at 17:48
  • $\begingroup$ @K7PEH: Ok I guess I should formulate it a bit weaker. For me it is clear that the bracket notation as discussed in the link above, where the bracked is around the physical quantity makes perfectly sense for dimensional analysis. However (and this is the weaker version of my comment) I don't see why writing the brackets around the unit itself makes any sense, for example $[\mathrm{kg}]$. For example people use it like this: $E [\mathrm{J}] = \frac{1}{2} m [\mathrm{kg}] \cdot v^2 [\mathrm{\frac{m^2}{s^2}}]$. $\endgroup$ – student Apr 20 '17 at 19:45
  • $\begingroup$ In my own experience with dimensional analysis, I would not write the equation has you have done. Instead it would be written like $[E] = [M][L]²[T]^{-2}$. Where, E is dimension for Energy, M is Mass, and L is length, and T is time. Note that constants are not included the the $1/2$ is left out. If you have not yet done so, Google Buckingham Pi Theorem to see how such dimensional analysis is used. $\endgroup$ – K7PEH Apr 21 '17 at 3:22
  • $\begingroup$ @K7PEH Ok, so you use one of the modern conventions (as discussed in the link), but that's not what I am asking about. $\endgroup$ – student Apr 21 '17 at 5:44
  • $\begingroup$ I couldn't find the use you describe in books or publications, do you have a reference? It might just be taking SI symbology too literally:"The value of a physical quantity Q can be expressed as the product of a numerical value {Q} and a unit [Q], Q={Q} [Q]". Of course, braces and brackets are supposed to be replaced by numbers and unit names, not enclose them, but the phrasing is highly misleading. $\endgroup$ – Conifold Apr 21 '17 at 21:38

It is a habit of text books and handbooks (of engineering branches as well as of chemistry and even physics) to indicate the unit of numerical values given in tables by column headings like "m [kg]" or "m in kg". The mathematically correct way is "m/kg" since if the mass is m = 3 kg, then the numerical value is clearly 3 = m/kg. But even when students are told this, I see often the aformentioned habit show through. This might be the reason for further [ab]use of square brackets. (It is not distorting. We know what is meant.)

  • $\begingroup$ I am personally involved in the accelerator physics community, and I would say that square bracket notation for units is the most common, closely followed by parentheses, in both internal documents/presentations, and in proper peer-reviewed papers. The particle physicists however tend to write /kg in their publications. $\endgroup$ – a20 Mar 27 '19 at 8:03

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