You can't be too careful, you know - and even when you are, you can still put your foot right in it.
As I did here with my last rant.
Who would have thought that aitches could be so contentious?
Well, in Northern Ireland they are: aitch is pronounced with a h sound, haitch, by Catholics, and without the h, aitch, by Protestants. It's a real-live modern shibboleth.
The story of the first shibboleth.
Long long ago, after yet another battle in the lands now known as Palestine and Israel, the winners wanted to find out which of the soldiers came from the enemy tribe. So what they did was to get every man to say shibboleth.
The winners' tribe could say the sh sound all right, but their enemy, who had no sh in their own language, could only say sibboleth.
So the plan worked very well - though it led to a lot of unpleasantness - but, as you may have noticed, failed utterly to solve the problems of the Middle East.
Shibboleths have been used in more modern times, too. In the Second World War the Dutch used the name of the town of Schevenigen to identify Germans (again, the sh sound was the give-away).
So shibboleths are any words which are special to a particular group. It's also come to mean a custom that's a bit out-of-date.
Shibboleth: this is a Hebrew word. Shibbόleth means ear of corn, as it happens, though that's got nothing at all to do with either of its current meanings.