What is tremolo on the classical guitar?
Most commonly, classical guitarists or even flamenco guitar guitarists will play a low note with the thumb and then three high notes consecutively.
The three high notes are most often played in the following order: ring, middle, and index. However. some guitarists will play middle, index, middle.
When performed evenly, the listeners mind will fill in the little space between the high notes where the thumb plays.
To perform tremolo the thumb would pluck. Then the ring finger, middle finger, index finger would pluck consecutively after that on the same note. When the speed of the performance increases enough the illusion of a steady and continuous melody appears.
Some styles of music require different types of tremolo with varying numbers of tremolo notes. Flamenco guitarists are famous for playing a five note tremolo.
Composer Robert Beaser's work Shenandoah uses a beautiful but highly irregular tremolo.