Why nav-tabs cannot be used with nav-stacked?

Question

Why this case is invalid ?

<ul class="nav nav-tabs nav-stacked" role="tablist">
 <li> ... </li>
 <li> ... </li>
</ul>

Answer 1

Only pills are stackable. Not tabs. See bootstrap docs:

http://getbootstrap.com/components/#nav-tabs

You can always achieve the same effect by styling the nav-pill and nav-stacked if you want stacked tabs. Although, stacked tabs sound counter-intuitive.

Answer 2

Only pills are stacked, because tabs have special left/right/top borders to look like tabs. They used to exist in 2.x as tabs-left and tabs-right, but they were removed from 3.x.

See my answer here:
Stacked Tabs in Bootstrap 3