In classical guitar, tremolo consists of three notes, whereas in flamenco guitar it normally consists of four notes.
Tremolo was invented in the 16th century, in order to imitate the effects of nature. Composers such as Paganini, Tárrega, Barrios Mangore or Eduardo Sainz de la Maza used this resource in their compositions, as well as many flamenco guitar musicians.
It is not the same tremolo and arpeggio. When the notes of a chord are played one after the other, we are making an arpeggio, named after the harp.
Tremolo, both in the flamenco guitar and the classical guitar, produces a very similar effect to that generated by other plucked string instruments like the bandurria or the mandolin. It is a repetition of the same note played normally in sixteenth or thirty-second note, creating an effect of continuity in the melody.
In order to obtain this effect of continuity, it must be played with proper speed, so that when tremolo is not playing, but the thumb accompaniment does, the feeling we can get is that the melody does not stop.
Normally, the technique is applied by the ring finger (a), middle finger (m) and index finger (i), which are those that play the melody. This movement must always be alternated with the thumb, which makes the accompaniment.
The most common way of reproducing the action of the right hand would be playing: p-a-m-i-p. Many guitar players use a different progression, particularly p-i-m-a, or p-i-a-m.
It is also very common to hear guitarists who apply it at full speed, although, as it has been mentioned before, it is difficult to get a homogeneous result where notes flow with harmony, and controlling the volume in every moment of the piece.
In flamenco, it is very common four-note tremolo, in such case it is added one more note, so the technique is somewhat different, usually p-i-a-m-i.
How to study tremolo
Tremolo, in both flamenco guitar and classical guitar, requires, as any other technique, hard work and dedication. Starting to play it slowly helps improve the ability of obtaining the right touch and the right sound power needed
Obtaining the adequate volume that highlights melody and modulates itself in an homogeneous form is the most important thing along with obtaining the right rhythmic precision and speed.
In relation to this aspect, an interesting resource to use is a sordine, it can be a sponge, chamois cloth or made with paper. In flamenco and classical guitar, a sordine isolates the melody of harmonics generated and helps to a better control of the technique.
In some video by Paco de Lucía, it can be seen how he uses a sordine in his flamenco guitar. A resource that, on the one hand, obtains the adequate volume in order to play in places such as a hotel room where it is not allowed to disturb. And on the other hand, it helps achieve the right power in the right hand, therefore it also helps a increase the volume in which we play.