A Violin Has One Bow and Four Strings. How Do You Play Two Notes?
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Sounding two strings at once on a bowed instrument requires the bow to touch both, which the curve of the bridge makes deliberately difficult, and the writing has to account for it.
What the technique requires
The player presses two strings with the left hand and draws the bow across both simultaneously. That is harder than it sounds because the bridge is curved, deliberately, so that a bow can reach each string separately without catching its neighbours. Playing two at once means pressing the bow in further and finding the angle where it contacts both evenly, which takes more weight and gives less control over tone. Intonation is also harder, since two notes must be in tune with each other as well as in tune absolutely, and any error is exposed by the interval beating.
What the instrument permits
The geometry sets hard limits on what can be written:
- •Only adjacent strings can be sounded together
- •So the available intervals are constrained by the tuning
- •One note is frequently an open string, which is much easier
- •Some fingerings are physically impossible to reach
- •Three or four strings cannot be sustained together at all
- •Those are played as a rapid roll across the strings instead
How composers use it
The technique supplies harmony from an instrument that is fundamentally melodic, which is why solo writing depends on it. Bach's works for unaccompanied violin are the extreme case, implying four part counterpoint from a single instrument by combining sustained pairs with rolled chords and with lines that leap between registers fast enough for the ear to hold both. Orchestral writing uses it more sparingly, to thicken a passage without adding players. Folk traditions across Europe use sustained pairs with one open string as a drone, which is the easiest version and the oldest.
The tuning that makes it possible
The intervals available depend entirely on how the instrument is tuned and the choice was made for exactly this reason. A violin, viola and cello are tuned in fifths, which spreads four strings across a wide range and means that a hand in one position can reach useful intervals on adjacent strings. A double bass and a guitar are tuned in fourths, which suits a different hand shape and different repertoire. Some folk traditions retune the instrument entirely for a particular tune, which puts the desired pairs under the fingers and makes passages possible that standard tuning would not permit.
What a player has to practise
The specific difficulties are well known to anybody who has attempted it. Bow weight must increase without the tone becoming crushed, and the contact point must be found reliably in performance rather than by adjustment. Both notes must be stopped accurately, and the left hand shape required is frequently uncomfortable. Tuning is checked by listening for the beating between the two notes and eliminating it, which is a skill separate from playing a single line in tune. And a passage of consecutive pairs requires all of that to be reset for every change.
The takeaway
Sounding two strings at once requires pressing the bow further into a deliberately curved bridge, which costs weight and control, and demands that the two notes be in tune with each other as well as absolutely. Only adjacent strings work, and three or four are rolled rather than sustained. Bach's unaccompanied violin works are built almost entirely on the technique.