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I know this might seem like an absolutely silly question to ask, yet I am too curious not to ask...

Why did "i" and "j" become THE variables to use as counters in most control structures?

Although common sense tells me they are just like X, which is used for representing unknown values, I can't help to think that there must be a reason why everyone gets taught the same way over and over again.

Is it because it is actually recommended for best practices, or a convention, or does it have some obscure reason behind it?

Just in case, I know I can give them whatever name I want and that variables names are not relevant.