School children around the world today generally use the same length ruler, being 30cm long. For the Yanks and maybe some British, they may still use the foot ruler.

I remember when  I was in junior school around Std 1, we did the whole switch from imperial measurements to metric, and I had to stop using my nice wooden ruler with the embedded brass strip, in exchanged for this new 30 cm plastic ruler with miniature “inches”.

My first thought of course was “why 30cm?” … that’s a strange number. It’s not called anything, it doesn’t divide nicely into 1 metre, so what’s the story?

I guess it was just some bureaucrat or production manager somewhere that decided to keep the ruler the same length more or less as the 12 inch rulers they were making the day before.

On the positive side, it’s good for drawing lines down the length of a typical A4 page. On the negative side, it’s useless in other regards … it’s too big to fit into normal pencil cases, and more-than-needed for drawing most lines needed in school geometry.

But the market eventually saw the need for something shorter, and rolled out these:

20 cm ruler

Amazon has lots of different ones to choose from. But try buying one here in South Africa, either retail or online, and you’re likely out of luck. I actually bought one from Amazon UK for my kid.

These rulers fit very nicely into standard pencil cases, and are perfect for drawing lines across an A4 page, which is done much more often than drawing down the length. They’re also the right size for doing school geometry… not too long, and not as annoyingly short as a 15cm/6 inch ruler.

These, like their bigger cousins, do not have a name beyond “20 cm ruler”, but its such a useful size (and I’m likely to refer to them a few times) that I have thunk up a name, being wroft, because the 20cm is “about” the distance from your wrist to the tip of your middle finger, so WRist tO Finger Tip  = wroft.

Hand – wroft

Also it does not exist as a word in English yet. And for a symbol, we can use the Coptic small letter shei, ϣ, which looks a bit like a double-u. It looks prettier in a prettier font than this.

Now what does all this have to do with the cubit?

Quite simply, this:

wroft from cubit

I would have preferred wrift to wroft but wrift is used as an acronym for other things.

If you do that sum on your calculator it comes out as 0.1999969358 which is really really close to 20cm …. especially on a ruler.