In some circumstances, a "triple stop" is possible, where three notes across three strings can be voiced simultaneously. The bow will not naturally strike three strings at once, but if there is sufficient bow speed and pressure when the violinist "breaks" (sounds) a three note chord, the bow hair can be bent temporarily onto three strings, allowing each to sound simultaneously. This is accomplished with a heavy stroke, typically near the frog, and produces a loud and aggressive tone. Double stops in orchestra are occasionally marked divisi and divided between the players, with some division of the musicians playing the lower note and some division playing the higher note. Double stops (and divisi) are common in orchestral repertoire when the violins play accompaniment and another instrument or section plays melodically. In some genres of historically informed performance (usually of Baroque music and earlier), neither split-chord nor triple-stop chords are thought to be appropriate; some violinists will arpeggiate all chords (including regular double stops), playing all or most notes individually as if they had been written as a slurred figure. However, with the development of modern violins, triple-stopping has become more natural due to the bridge being less curved. In some musical styles, a sustained open string drone can be played during a passage mainly written on an adjacent string, to provide a basic accompaniment. This is more often seen in folk traditions than in classical music. Vibrato Vibrato is a technique of the left hand and arm in which the pitch of a note varies subtly in a pulsating rhythm. While various parts of the hand or arm may be involved in the motion, the result is a movement of the fingertip bringing about a slight change in vibrating string length, which causes an undulation in pitch. Most violinists oscillate below the note, or lower in pitch from the actual note when using vibrato, since it is believed that perception favors the highest pitch in a varying sound. Vibrato does little, if anything, to disguise an out-of-tune note; in other words, misapplied vibrato is a poor substitute for good intonation. Scales and other exercises meant to work on intonation are typically played without vibrato to make the work easier and more effective. Music students are often taught that unless otherwise marked in music, vibrato is assumed. However, it has to be noted that this is only a trend; there is