When you’re playing on all white keys and you skip a note, you’re playing a third. Some of them are major thirds, some of them are minor thirds, and it’s easy to tell which is which. The major thirds have two black notes in between them.
Major third, and the minor thirds only have one. You can tell this is a C major, a D minor, an E minor, an F major, a G major, an A minor. And this one here has a minor third in the bottom and a minor third on the top.
It’s diminished.
