In bluegrass music, the "dobro" refers to a resonator guitar, played across the lap - with a steel bar sliding on the strings. This is different than resonator guitars that are played in a traditional style - with or without a slide - like in certain styles of blues music. The spider bridge is the iconic resonator sound of bluegrass music. The other popular style - used for blues music - is the biscuit bridge. Less common is the tricone resonator guitar, using 3 smaller cones for a more complex sound.
Recording King RR36s-vs Maxwell - This square-necked spider cone resonator guitar is well suited for playing bluegrass music. The square neck is necessary to handle the tension from the GBDgbd tuning that this style of resonator guitar uses.