I understand time signatures in theory, but not in practice. I comprehend what they are and what they mean, but they don't make sense to me. I don't understand what they do or why they're necessary.
Like I get it, the bottom number tells you how many beats are in a whole note. Most commonly 4, but sometimes 8 or something else.
The top number tells you how many beats are in a measure. In common time, that's again 4.
But mathematically, wouldn't that mean 3/4 time is the same as 6/8 time? They're equivalent as fractions. But when I ask a musician they don't explain it, they just tell me "No, they're different. That's 3/4, this is 6/8." And usually with this undertone like "Are you stupid? This is so obvious, what part are you not getting?"
From what I gather, 6/8 time is usually done in pairs of triplets. So it does sound rhythmically different than 3/4. But no one ever tells you that, not even when the concept is introduced in textbooks on music theory. They just tell you "and this is 6/8 time," as if you're just supposed to know how and why that's in any way different from 3/4 time...