Nesting Tabs inside a Tab?

I'm a bit confused about whether its possible to nest tabs within a tab. I've created some nested tabs ok but I wanted them to sit underneath a tab. I thought if I had a tab horizontal as a "root" and then a series of nested tabs under it. The nesting would break once I created another Tab horizontal? Is this how it works or is it not possible to nest tabs within tabs?

The tab nesting does not let you nest tabs within any tab you want. It creates its own "More" tab or menu depending on the position and display mode. In the case of the canvas menu it will create a dropdown button where the nested tabs can be selected. In the case of the main middle position it'll create a More tab with your nested tabs in it.