In Aus, there are a subset of 31cm pencil cases. AFAI can see, the main benefit is that you can fit a 30cm ruler inside.
Also 
If you aren’t a little bit careful, your ruler may eventually snap or be snapped. From when I was a teenage boy, I remember a lot of short pencil cases had half-a-ruler inside with all the other garbage.
It occurs to me: Despite the variation in short rulers, I’ve never seen a meterstick that was anything other than exactly 1 meter long, without any slack on either end.
I haven’t looked one way or another, but I assume it’s because that level of precision is not expected on a yard/meter stick, so if it’s off a sixteenth or an eighth of an inch or metric equivalent, it makes no difference.
My steel meter rules are flush at the zero end, but have slack on the 1m end. My wooden one has no slack (and brass end-caps to cut down on wear)
Super cool: Starrett makes a 150mm scale for machinists in Millimeter Land.
It’s 19mm wide, while the 6" variant I use is 3/4" wide (19.05mm), so they are all but identical in all dimensions.
It’s marked in 1/2 millimeter increments, and it is exactly 150mm, within the precision allowed by manufacturing.
So it’s almost certain that machinists the world over carry a small metal ruler about 6" or 150mm in their shirt pocket, used for everything from scraping to prying to scribing to screw-turning to … measuring.
What about that is surprising or super cool? The desk I am sitting at right now randomly has a ruler on it marked “STAINLESS”, 15 cm with the first 5 mm graduated in 0.5 mm intervals, then 1mm; the other side 6 in in both 1/20" and 1/32" with a little bit of 1/50" and 1/64". Width seems to be about 19.3mm
For maximum convenience, one of our neighbo(u?)rs to the north has developed this handy ruler with metric ticks labeled with fractional inches. Something for everyone!
Unless you have used one of these Starrett scales (or similar ones made for the industry) you wouldn’t understand. They are solid, hefty, virtually unbreakable, with the lines etched deep, made of hardened steel. It’s a fine instrument made for precision work.
And something surprising or super cool to me doesn’t necessarily meet the same standard for anyone else.
Note the date of that article 
For the rest of the world, when we use fractional inch measurements, it’s always in powers of 2 (1/2,1/4, 1/8, 1/16…1/128). It’s a system that has its own elegance and can be easily worked with, though that elegance evaporates beyond the inch level.
Oh, I am not sure my ruler is not a cheap piece of shit, even though there is nothing obviously wrong with it, I mean, it is a literal no-name desk ruler, not a calibrated precision instrument. I thought you meant it was surprising that a ruler would be only 6 inches long.
I think some old (not sure how old) American electronics had some connections spaced in 1/1000s of an inch?
I once heard a story about an unauthorized Soviet copy of I forget which American computer (VAX??) where they laid out the circuit board using 25 mm per “inch”. Which worked OK until they tried increasing the clock rate past a certain point…
Nah. Just pleased that if I need to machine in mm, I can have a similar handy tool.
And the inexpensive thin stainless steel ones definitely have their use–I wouldn’t imagine using a machinist’s scale for pencil sketching, measuring cloth, general use in a classroom, or all the other stuff that a desk ruler does. It’s too clunky for that.
0.100 inch is still pretty standard for a lot of things where small size isn’t important. Through hole integrated circuits and breadboards for examples.
My favorite use for a metal machinist’s rule is diffracting light. You can literally use a ruler marked in millimeters, to measure the wavelength of a laser to a precision of about a nanometer.
Wikipedia says:
“The introduction of the thousandth of an inch as a sensible base unit in engineering and machining is generally attributed to Joseph Whitworth, who wrote in 1857…”
Whitworth was doing what was, at the time, precision engineering, where accuracy to .0001 was required in measurement, but the unit of measurement became widespread, even when accuracy to only 0.1 was all that was required.
Manufacturing dimensions in Australia which used to be specified in 'thou are now all in mm (~4 thou), which includes stuff like 3832 mm – manufacturing workers generally have trouble with ‘decimals’ so you don’t write things like 3.832 m.
I have two wooden rulers on my desk. One is an absolutely ancient drafting ruler. Beveled and you can see the channel that used to hold a metal strip along one edge. It measures end to end, precisely. The other ruler is a cheapo giveaway from my union that overshoots an exact foot by about a millimeter at each end.
The big airplane company I use to work for required every measuring device to be certified, even Starrett 6" rulers. All rulers up to 12" in length required the 1" mark and the mark 1" from the other end must be within .001" from the end of the ruler. The 4 corners must be at 90 degrees, + or - .01 degree. Rulers that do not meet these requirements must be destroyed. I have a couple of these, they are close enough for me. QA requirements for these types of rulers required all measurements to be made from an inch mark, not the ends.
I’d say that if the 1" mark is within 0.001" from the end of the ruler, that’s a very poor ruler indeed.
Our QC person worked for Lear, which supplied carpet to auto makers. They used tape measures, and had to have them in their calibration program. They are not high precision instruments.
In that case the ruler should be hotdog-shaped, with the ends round to keep people from being tempted to use them.