Brothers of steel

"Dobro" is actually a brand name for a type of instrument (a "resonator guitar") invented by the Slovenian immigrant John Dopyera and his brothers in the 1920's (their innovation thus being named after them, "DoBros"). Standard guitars had always been the quietest instrument in any band, by the laws of physics—it couldn't compete with the ring of a banjo or the wail of a fiddle or even the double-string courses of the mandolin. John and his brothers had the idea of adding a metal plate to the front of a guitar, giving it a deep resonance.

Other companies began making similar instruments, so while all Dobros are resonator guitars, not all resonator guitars are Dobros.

To add to the confusion, there are two types of resonator guitars, one with a rounded neck that can be played like a normal guitar (such as Eric Clapton sometimes plays) and another version with a bulky square neck and strings raised so far above the fretboard it can only be played with the instrument sitting in your lap, using a steel bar to slide up and down the strings. The latter type lap-style instrument is what people usually mean by the generic "dobro."

On a regular guitar the notes are stopped at precise locations by frets (so you're playing say a G or A or whatever note exactly). On a squareneck dobro, because of the high strings, a steel bar is used to slide anywhere on or between the notes, making it tricky to play a note exactly. The tradeoff is that using a bar eliminates the friction of fingers and the sound can glide effortlessly up and down between notes.

The dobro can create a rough, rootsy vibe or a slippery, sometimes haunting sound very close to the human voice. Dobros are prominently featured in folk, country, and bluegrass music, of course, and in films such as O Brother Where Are Thou, but soundtrack composers also use its odd timbre in unexpected ways, such as in horror movies and gangster films. Director John Sayles said he loved the dobro sound in his movie Matewan because the dobro's notes "bend into a question at the end."

Next time you hear an unusual but sliding sound in a movie soundtrack you might have the Dopyera brothers to thank.