Musicians pull two strings at once, doubling up on the vibration. The vihuela, a stumpy six-string instrument with a curvaceous back, looks something like a lute. It takes part in the rhythm, too. Every mariachi band needs a guitarra. Most use the standard six-string Spanish variety.
Mariachis tend to load up on violins.
Trompetas, too, get melodious in mariachi music. They blasted their way into modern-day mariachi music in the 1930s and '40s. Arpas can go rhythmic or melodious, but harps don't take part in mariachi music as much as they did in olden times.