@ddunbar had posted the lefty loosey, righty tighty bit as a mnemonic for prop positions in Rugby football, and that reminded me of the excellent mnemonic for unscrewing jars, screws, nuts etc. I got a whole bunch of comments about how it should really be clockwise or anti-clockwise, or is dependent on spanner position or viewpoint and that got me thinking about the general applicability of the advice.
My first thought was that the mnemonic was generally applicable in all cases where a right hand thread is used, which is by far and away the most common. In some cases a left hand thread is used when the natural motion of the joint is anti-clockwise (as in the case of the pedal on the left-side of the bicycle) and would lead to loosening of the joint. Ok, exception number one found and agreed upon.
But is that the only exception I asked myself, and why? Well the answer is simple, but as always in these cases, took me some time to arrive at due to my natural bias: as a right-handed person. In most cases where I would have to turn something to loosen or tighten it, I would place my right-hand thus that the natural turning motion of my hand to give maximum torque would always lead me to turn my hand to the left in loosening operations, and to the the right during tightening. The default spanner position and point of view plays no part in it, because to do the opposite would force me to put my hand in an unnatural position i.e. upside down and that would also mean that I would not be able to exert maximum torque. Job done, so I thought; but then I realised that this is only valid for right-handed people and for left-handed people it would be exactly the opposite. So there you have it, exception number two.
So, lefty loosey, righty tighty is valid for right handed people and right-handed threads. Still, you have to admit that as a mnemonic it did its job superbly...