The tempo tells you how quickly the music should be played. Usually, tempo is expressed in beats per minute bpm for a specific beat type. This means 50 crochet beats in a minute. To will let you play at the correct tempo, which you can use a metronome. These can be either analogue or digital but the feature is an analogous: it ticks or bleeps on each beat at the set tempo. Metronomes can be especially effective for orchestras or bands to keep every person in time with one another. In many pieces the tempo changes. This is customarily communicated using musical terms. Scales have key signatures which mean bound notes are always sharpened or flattened. At the start of a chunk music, sharps or flats not both are written on the employees lines. This applies the sharp or flat to all the cases of that note, despite pitch.