Unison or perfect unison (also called a prime, or perfect prime) may refer to the (pseudo-)interval formed by a tone and its duplication (in German, Unisono, Einklang, or Prime), for example C–C, as differentiated from the second, C–D, etc. In the unison the two pitches have the ratio of 1:1 or 0 half steps and zero cents. Although two tones in unison are considered to be the same pitch, they are still perceivable as coming from separate sources, whether played on instruments of a different type: play unison on C, piano and guitar (help·info); or of the same type: play unison on C, two pianos (help·info). This is because a pair of tones in unison come from different locations or can have different "colors" (timbres), i.e. come from different musical instruments or human voices. Voices with different colors have, as sound waves, different waveforms. These waveforms have the same fundamental frequency but differ in the amplitudes of their higher harmonics. The unison is considered the most consonant interval while the near unison is considered the most dissonant. The unison is also the easiest interval to tune. The unison is abbreviated as P1. However, the unison was questioned by Zarlino as an interval for lacking contrast and compared to a point in geometry: In orchestral music unison can mean the simultaneous playing of a note (or a series of notes constituting a melody) by different instruments, either at the same pitch; or in a different octave, for example, cello and double bass (all'unisono). Typically a section string player plays unison with the rest of the section. Occasionally the Italian word divisi (meaning divided, abbrev. div.) marks a point where an instrumental section, typically the first violins, is to be divided into two groups for rendering passages that might, for example, include full chords. Thus, in the divisi first violins the "outside" players (nearer the audience) might play the top note of the chord, while the "inside" seated players play the middle note, and the second violins play the bottom note. At the point where the first violins no longer play divisi, the score may indicate this with unison (abbrev. unis.).