What is the theorbo, you ask? It's a "plucked string instrument of the lute family, with an extended neck and a second pegbox," says Wikipedia, helpfully. Only I think "extended" is probably underselling it a bit.
Sure, it sounds great (as the following setting of Santiago de Murcia's "Folias Galegas" will attest). But you know what's really great about it? The way it looks (especially as modeled in that fantastic blog image and in this fantastic YouTube clip by Luteduo's Anton Birula).
Make sure you stick it out until at least the 2:20 mark, because there's some cray cray key changes going on right about then.