With an electric bass, this is obvious: they're the same shape. In a blurry picture of a rock band, it's hard to tell who's playing which. The bass has bigger tuning pegs and fewer strings, unless it's a six-string bass in which case ... well ... it still has bigger tuning pegs.
Traditionally, a bass just has the lowest four strings instead of all six, but they're tuned an octave below a guitar's.
A stand-up bass? Not as easy to spot the resemblance, but the tuning's the same.
Other instruments that look like fiddles -- cellos and violas -- are big fiddles. They're tuned in fifths, so if one string is a D, the next higher string is an A.
A bass? It's not. To see why, let me do a quick side-trip.
Violin family instruments are all tuned the same, just scaled up and down. The viola adds a string below the violin's lowest -- a C -- and removes the violin's highest -- an E. It's just a violin tuned down a fifth. If they had frets, a violin would be a viola capo'd at the seventh fret.
A cello? It's just an octave viola. A viola is miniature cello, pitched an octave up.
Mandolins use this same scheme. A mandolin is tuned like a violin, a mando-cello like a cello, though you hold it like a mando.
Octave mandolins (which you'll also hear called bouzoukis, though bouzouki players will argue with one another about that), sit right in the middle -- they're like a mando-cello without the lowest C, but with an E added as a high string. Banjo-mandolins are mandos built with a banjo head, and tenor banjos, which pop up in Irish and contradance bands, are tuned just like cellos.
Play one, you can play them all.
You can reason your way from here, and people do. There's fiddle analogue to the octave mandolin called the baritone violin, but I've never seen one. I'm not sure why. However, lots of fiddlers I know have owned five-string violins, which add a low C and are just hybrid violin/violas. Most makers put these on a viola body, but I've seen one on a violin body, too. Buying strings must get expensive.
(You can also reason your way from here badly. A banjo-guitar and a banjo-uke are a guitar and a uke on banjo bodies. Noisy but sometimes useful. A mandolin-banjo is a five-string banjo on a mandolin body. Eeeew. It's a distant relative to the backpack mandolin you find for sale in music stores: "Looks like a mandolin, sounds like a backpack.")
But a bass? It's not really in these families at all. It's not a bass violin, it's a bass viol.
Viols are extinct.
Just as Homo sapiens replaced Neanderthals by the end of the Pleistocene, viols died out at the end of the Renaissance, replaced by the smarter, bigger-brained violins. You can still see fossil remains of viols at early music concerts. You can pictures of viol players, holding their instruments, in old Dutch-master oil paintings, by guys like Fritz Hals. They all look like you wish you played in their band.
At first, the viols in those paintings look like violins and cellos, but then you see that there's something a little off. The sticks on the bows aren't flat, they're curved. The necks have frets, usually tied on with strings. There are too many pegs -- the little ones have six instead of four. not quite the shape you're expecting. It turns out, they were tuned in fourths. They're guitars played with bows.
If these were by lesser painters, you'd say, "They aren't musicians, so they're painting what they think they saw." Uh-uh. Look at those faces. They're real. So are the instruments.
They saw better than you have. Look where the instrument's neck joins the body. Instead of a right-angle, abrupt corner, the body flows into the neck in a graceful curve. That's a viol. And a bass? Look at the neck-body join.
Most of the viols were replaced, but the bass violin never took off. If you play a mando-cello, you'll see that the reaches on a neck that big make the interval of a fifth between strings hard to handle. Even in mandolin orchestras, you'll often see the easier-to-get-around guitar taking up that slot. And a bass? A fifth is just too big an interval between strings at that size. The bass viol was doing the job, so it stuck around.
The bass is a living fossil, a musical ginko tree.
