Watch the Hammer-On online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
The granddaddy of all legato technique for fretted instruments is the hammer-on. Whether it's a single instance or a string of hammer-ons, the basics are the same: A picked or fingerpicked open or fretted note is followed by a fret hand finger that comes down hard on the fretboard activating a new fretted note. When the technique is developed, it's a powerful technique that has deep potential and also serves as a basis for many more legato-based techniques.
Looking for inspiration? Allan Holdsworth may be to the hammer-on what Frank Gambale is to sweep and economy picking. That said, Joe Satriani is a hammer-on super hero as well. With a ubiquitous technique such as hammer-ons, source material is not hard to find given the gaggles of great players we have out there today. To help get your hammer-ons sounding clean, be sure to check out my Guitar Gym: Hammer-Ons & Pull-Offs