## 21 April 2024

### Use the solidus, not the obelus, to indicate division

I encountered the plural "obeli" in a crossword puzzle today and had to look it up.
The word "obelus" comes from ὀβελός (obelós), the Ancient Greek word for a sharpened stick, spit, or pointed pillar. This is the same root as that of the word 'obelisk'...

The form of the obelus as a horizontal line with a dot above and a dot below, ÷, was first used as a symbol for division by the Swiss mathematician Johann Rahn in his book Teutsche Algebra in 1659. This gave rise to the modern mathematical symbol ÷, used in anglophone countries as a division sign. This usage, though widespread in Anglophone countries, is neither universal nor recommended: the ISO 80000-2 standard for mathematical notation recommends only the solidus / or fraction bar for division, or the colon : for ratios; it says that ÷ "should not be used" for division.

You learn something every day.

1. The things one doesn't think about !

Seems obvious when looked at now, the sharpened spear smashing through ... a tree, or an iced bun (yum) leaving two parts or a split trunk ... yes, division.

And the plus sign, that upright standing 'once was a multiplication sign', can be viewed as combining the split pieces of that trunk to form something more, such as a fence or a house.

Sadly, combining the split asunder parts of the iced bun will only give you less, one half sized piece for your friend and one half sized, and slightly smaller, piece for you.

1. Not to mention that it very much puts you in mind of the way we are usually taught to write fractions when we begin learning them!

2. In cases where you want to indicate an algebraic expression via text, the exponentiation symbol can be ** or ^ . A one character ^ is definitely better. ** must be a holdover from a time when people had limited characters available, ie, less than the char set of a typewriter!

3. AIR, the '*' was a multiplication symbol (instead of 'x') and the '**" was the exponent symbol (not confuse withe the '*' multiplication symbol. Also, the '/' symbol was called a 'divide sign' and the '#' was a 'pound sign' (as in weight).

4. An obol is also an ancient Greek coin. It comes from the spits that were used to roast meat which were exchanged as a form of "utensil money" before coins were developed. https://en.wikipedia.org/wiki/Obol_(coin)