Intervals are named in a funny way. The logic eludes many beginners, but there's actually some sort of pattern at play.

I really think you should learn this early on, and not later, because it'll save you some serious headaches down the road. However, this is the least fun music theory ever gets in my opinion - it's lots of rules that we can't really apply till later.

I hope that's not too discouraging. But I promise - if you spend an hour working on this now, your future self will thank you.

Here's what I cover in the lesson:

How "letter classes" are counted
Why intervals are "major" or "minor"
What a "perfect" interval is
How augmented and diminished intervals work

Everything here is covered in more depth in Chapter 2 of The Chord Progression Codex, which you can read for free. To train your ear on these sounds, see my name-your-own-price Guitar Interval Course.