There are quite a few different schematic symbols for capacitors, some geographically mandated (Euro cap symbols differ or have differed from U.S. symbols), some according to year, others according to company drawing standards. In 1944, caps were drawn as two straight lines. The curve of one plate appeared during the late 1950s, early 1960s. The curved side usually meant negative lead for electrolytics or "outside foil" for axial-lead paper or plastic dielectric caps.
The smallest electrolytic (polarized) capacitor that I recall seeing is a little 0.47µF (and there may have been a 0.1µF) tantalum that I have in my stock. If you compare the same value electrolytic against a non-polarized version, you'll find a LOT less series inductance and ESR on the electrolytic, which could be critical in some applications.
I was once told by an older, more knowledgable engineer that a non-polarized cap can be made by connecting two electrolytics back to back ( minus lead to minus lead ), however, the elctrolytics must have first been "made" by connecting them to a DC voltage that is within 25% of their rated voltage. In light of the above post, I think that might not be the best idea.
I can't quite see how connecting electrolytic capacitors back to back can be acceptable or OK.
Surely one of the capacitors must be incorrectly polarised and so would be in exactly the same position as if it were a single capacitor incorrectly biassed. The only way this wouldn't be the case would be if the reversed capacitor were a dead short or somehow generated a reverse voltage, I suspect that neither is the case.
The only way that I can see it working is if the incorrectly biassed one somehow looks like a very low impedance for some chemical reason and only not gassing to distruction because the other capacitor is limiting the current flow through it.
So the reason why the capacitor doesn't fail is not because it is 'OK', but because you are 'just lucky' that some other effect has occurred. Personally I'd rather keep within the manufacturers recommended operating conditions.
Well, that means that if I need to replace my 240v induction motor's starting capacitor, I can just use two electrolytics back to back. Anyone have the gonads to try that one and stay alongside the motor to see how well it works?