I've just realised something about gMail which might be something new but it's new for me - hence this post.
Set up a new label in gMail such as Music / Amplifico or Tech / TWiT
Then look at the Label List - you'll see a wee cross next to the top level label. Click it. And presto! The second level label appears.
Now add a secondary label such as Music / Hollow Horse or Tech / Dvorak. You'll see each of these are now "nested" out of the Music or Tech labels. Neat huh?
If this is new or not spotted before then yay me. Otherwise just put it down to age. A senior moment! lolz.