The guitar is a very expressive instrument. Unlike the piano, it has an added advantage in that we can control the sound of a note in many ways after we play it. Some simple expressive techniques are commonplace in most guitarists' playing style. The most common ones are known as hammer-ons, pull-offs, slides and bends.
As the name suggests, a hammer-on is a technique where we play a note on the guitar, and, while this note is ringing, fretting another note on the same string by 'hammering' on the string with another finger. The new note will then sound, even though the string is not plucked again.
The hammer-on needs to be quick and strong. If you pick the first note and then slowly press your finger onto the string for the second note, the string will be muffled and a musical note will not emerge. The string needs to strike the fretboard with strength and conviction. Hammer-ons are denoted with an H above or below a slur ( or ) which joins the two notes (the original note and the hammered-on note).
A pull-off is the opposite of a hammer-on. Where a hammer-on allows you to play a higher note from the one originally played, a pull-off allows you to play a lower note than the one originally played. So we play a fretted note as normal, and then quickly pull this fretted finger off the string in a downward motion, sounding a second note without making a second finger (or pick) stroke.
The second note, however, will usually need to be fretted as well (unless it is an open string), so preparation for this requires us to fret a note behind the original note before the original note is played. Pull-offs are denoted with a P above or below a slur ( or ), similar to the hammer-on.
We can produce a slide by picking a fretted note and then sliding our fretting finger up or down the string to another fret. When the destination fret is reached, this new note will sound. In order to produce an effective slide, constant pressure is needed on the string throughout the length of the slide. Slides can be as short as a single fret through to the entire length of the fretboard. Slides are denoted with a diagonal line with sl. written above it.
String bends are an important part of overall playing ability. Put simply, a bend is where we bend the string upwards using the fretting finger after playing a fretted note. The note then rises in pitch. Bringing the string back down again will make the note fall in pitch until it returns to the original note. Supporting fingers must be used when bending, so that you have enough strength behind the string to bend it. Two or three fingers are commonly used to support the finger doing the bending.
Bends can be small or large, though it is not very common for a bend to exceed 2 semitones difference in pitch. Bends can also create notes smaller than a semitone, which are normally used to add expression to a note, rather than adding to the melody line of the song. Bends are denoted with an arrow and normally have a fraction or number above it. This indicates the size of the bend (ie. how far to bend) in whole tones.
Here is an example using all the techniques described above. It is the theme from 'Coinleach Glas an Fhomhair' by Clannad: