Is it a guitar?

Bob-Dubery

The last couple of years I've seen a few things that are variations on the theme, and seem pretty guitar-ish but are arguably not guitars.

Watch this

Now Eric Bibb (lead vocals and hat) is certainly playing a guitar. But what's that that John Doyle (the left-handed player) is playing? Looks prettty much like a guitar, but TEN strings, in paired courses like a 12-string.

Well... Keef plays five-stringers, doesn't he? So maybe Doyle had a chat with a luthier and said that he wanted something somewhere between a twelve-string and a five-string. Dunno.

That's from the most recent Transatlantic Sessions. That series of TS also features Sarah Jarosz, who can be seen playing what looks like a way nice arch-top, but count the strings.... EIGHT. In pairs. So sort of like a bozouki or a mandocello or something.

Guitars didn't always have six strings - or even six COURSES of strings. There's an EIGHT string stradivarius guitar (four courses) on show at the Ashmolean museum in Oxford (which maybe sets a precdent for Sarah Jarosz's instrument).

Bob-Dubery

Here's Sarah Jarosz with that arch top wotchamacallit

And there's more! She's got an unusual banjo as well...

Got the usual extra string that starts up at the fifth fret, but another FIVE. That's a little unusual.

Tim O'Brien has some unusual instruments as well IIRC. Interesting that these players will get themselves an instrument with some kind of modification that they feel offers them an advantage.

On Martin Simpspon's latest album he plays a fretless banjo.

Jack-Flash-Jr

I think the output/end result is probably the way to define it... if it's sounds like a guitar then it is one. If it sounds like a banjo then it is one. A bass does not sound like a guitar therefore it isn't one.
IMHO that 10 string is definitely a guitar...