Guitar Hammer On – An already ringing note is changed by placing down hard on another fret

(Pronounced ham-er on)

Note: A guitar hammer on is the opposite of a guitar pull off.

To remember what a hammer on is, use the following mnemonic:

You don't have to use a hammer on (hammer on) the guitar; just press your finger onto the fretboard to cause a note to sound.

An already ringing tone is altered by hammering/forcing your finger to hold down the string closer to the body of the guitar as follows.

Imagine only one string on a guitar. Place one finger on the fret bar at position 1 and pluck the string at position 2.

Now hammer/force on another finger closer to the body of the guitar at position 3. 

Note: If you don't hammer on the finger fast, with speed, you won't hear the note very clearly. 

The reason for using hammer ons is that you get a slightly different sound than gently putting your finger at position 3 and then plucking the string at 2 again. It also allows you to pluck another string at position 2 at the same time creating some more interesting sounds and speeding up your playing.