String crossings
To move the bow from one string to the next, the whole arm changes level — like a lift stopping on a different floor. Get this smooth and quiet and a lot of music opens up; leave it jerky and every scale and arpeggio fights you.
Four levels, and the one in between
Each string sits at its own height of the whole arm — elbow and upper arm high for G, dropping for D, lower for A, lowest for E. Between any two there's a level where the hair touches both strings at once — you pass through it on the way across (and you'll use it for double stops in Stage 6).
How to play it
Slow, in the middle of the bow. Watch your elbow — it should lift and lower between the two levels. If the elbow is frozen and the wrist is flapping to reach, reset.
The arm leads, the forearm rotates
The impulse comes from the upper arm moving up and down — that's what changes strings. But the fine, fast version adds a small forearm rotation — the same movement as turning a doorknob — plus a slight wrist adjustment: higher string = higher wrist, lower string = lower wrist. A little rotation replaces a lot of arm movement, so a fast crossing passage becomes a small operation instead of a big one.
The keystone: cross early
The single most useful rule: the bow starts moving toward the new string while it's still playing the old one. When you want the note to sound is one thing; when the bow has to move is always a little before that.
Cross too late and fast passages feel clumsy, and slow ones get an unwanted accent at every crossing. The fix, and it suits your ear for timing: play the note before the crossing and the note after it as a double stop — both strings sounding together — then separate them again. Start slow and deep in the string, speed up toward tempo.
How to play it
Bow both strings at once, in the in-between level. Deep, slow, even. Then play them one at a time and notice the arm is already at the right height.
Try it
Try
Open D and open A, alternating, sixteen slow crossings. First exaggerate — big arm-level moves, watching the elbow. Then shrink it: same crossing, but tiny, from the forearm, elbow almost still. Then a one-octave G scale slurred two-per-bow, letting the D→A crossing happen smoothly inside the bow.
The usual mistake
Two, and they're opposites. Moving too big and too late — a lurch right on the beat that bumps the sound. And flapping the wrist to reach a string while the elbow stays frozen, which never quite gets there and tires the hand. The arm sets the level, early and smoothly; the wrist and forearm only finish the move.