How long a sound is played for.

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If you use beats to decide upon your duration, you must first set a tempo so you know how long each beat lasts.

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In school we have been learning that music is a combination of sounds and silence and work with these note (sounds) and rest (silence) durations:

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You could have a go using the rhythm randomiser to practice clapping rhythms.  You can change the setting so it shows only the notation you understand so far.