A tremelo can be defined as being a ‘trembling’ or ‘quivering’ effect – this usually takes one of two forms. One is where a single note is rapidly bowed repeatedly and the other is a a fast oscillation between two notes (rather like a trill) – with the gap between the two notes being wider than a trill.
A tremelo might be rhythmically even, or unmeasured (which can also be written as tremolando). The unmeasured version would be used where a composer or arranger instructs the notes to be simply played as quickly as possible – this technique is used often when adding strings to pop and rock tracks and can add a feeling of tension to a piece of music. Tremeloes can also be given an accent at the beginning (which means the first note of the tremelo is emphasised to give it a ‘kick’) – this can convey a feeling of tension even further and is particularly effective with a full string section rather than writing for single strings.
In string arranging, a tremolando can also be used very quietly – and if the whole of the string section are playing this way at the same time, it can give a rumbling feeling of something imminently about to happen – so instead of having a chord held, it can lend a sense of anticipation and of course add all important variety to string arrangements.