Paul H. Jacobson, Luthier

So why does the bass-E string need more vibrating room than the treble-E string anyway?  It's because the "string excursion"—the distance the string needs to travel when vibrating—is greater with the bass-E string, making it more susceptible to adjacent-fret buzzing.  The bass-E string should be about 1mm higher at the 12f than the treble-E string to optimally overcome this.

In this picture you can see clearly how the fingerboard edge on one of my guitars falls off toward the 19f on the bass side—the twist in the plane—to raise the bass-E string.  Notice, however, that the treble edge falls off toward the nut as well.  This is to distribute some of the overall twist-plane to the treble side in order to reduce the amount of fingerboard thinning between the 12f and 19f on the bass side.  Ramirez is noted for use of the bass-side twist-plane (which is where I got the idea years ago).  I haved encountered one guitar in my career in recent years, made in Germany by a luthier whose name I cannot recall, that had the twist-plane all on the treble side.  As far as I know, my guitars are the only ones made anywhere that use a combination bass/treble twist-plane.

If a builder doesn't use the twist-plane concept, there are only two other alternatives: settle for inadequate Eb-Et differential (or none at all), or; make the saddle higher on the bass side.  There's just one problem with raising the saddle: because of the geometry of guitar construction, to get a full 1mm Et-Eb differential, you must make the saddle 2mm higher on the bass side.  This is not just visually uncomfortable, it produces unacceptable torque stress on the bridge.  So most guitars end up with a string action that is too high on the treble side and not high enough on the bass side.  This causes your lightning-fast passages on the Et string to get slowed down because the Et string  is so hard to fret and your Eb string to buzz big-time when you push for heavy dynamics.

My combination bass/treble twist-plane fingerberboard is the best solution I know of for these issues.