In Lesson 4 and Lesson 5 of the Audanika Harmony Theory Blog, the concept of intervals and their names were introduced. We learned in Lesson 5 that a six semitone interval was a little different from the rest. Lesson 6 briefly explains why this is and how it is named, in a nice bite-sized chunk that even my attention span can cope with.
If you haven't been following the weekly lessons you should go back to the beginning and start! For those of you as jaded as I am, you might expect that Audanika's Harmony Theory Blog is just trying to up-sell you on Audanika's SoundPrism Pro. This is however not the case at all; apps are chosen for their instructive value, and other music apps are featured in the blog much more regularly than their own!