In music, intonation is the pitch accuracy of a musician or musical instrument. Intonation may be flat, sharp, or both, successively or simultaneously. In vocal music, intonation also signifies the singing of an opening phrase. Interval, melody, and harmony The lower or upper pitch of an interval may be sharp or flat, or both pitches of an interval. If the lower pitch is sharp or the upper pitch is flat, the interval may be said to be flat given that as a whole it is too narrow; while if the lower pitch is flat or the upper pitch is sharp, the interval may be said to be sharp given that as a whole it is too wide. Intervals are conventionally measured from the bottom, as such in an interval that is too wide the upper pitch is thus sharp. Intonation exists within the context of musical temperament, of which there are several types. However, the interval itself may be in tune, in relation to itself (i.e. both notes of the interval are in tune in relation to each other), but flat or sharp as a whole and thus both notes of the interval are out of tune. Notes within a melody can be either sharp or flat, or in tune, in relation to the other notes in the melody or other pitches sounding at the same time. Strings With fretless string instruments such as violins or cellos, intonation depends on the exact places the musician's fingers press the strings against the instrument's fingerboard, as well as any pull or push the musician exerts on the string, either along the string's length or perpendicular to it. The pleasantly "alive" sound of a large string section results from the amount by which each stringed instrument is slightly out of tune. Fretted instrument intonation Several factors affect fretted instrument intonation, including depth of the string slots in the nut, bridge saddle position, the position of the frets themselves, the bending stiffness of the string, and the technique of the musician. On fretted string instruments, pushing a string against a fret—aside from raising the string's pitch because its effective length is reduced—also causes a slight secondary raise in pitch because pushing the string increases its tension. If the instrument doesn't compensate for this with a slight increase in the distance from the bridge saddle to the fret, the note sounds sharp. Playing technique has