Interval size is the distance between the two notes in an interval as measured by the number of lines and spaces, including those on which the notes are placed, between the notes on a staff. Every interval can be defined by its size and its quality. Because interval size is measured on the staff and not in terms of the difference in pitch, it is not affected by any application of accidentals to either or both of the notes, although accidentals will affect the interval's quality. For example, every third spans two lines and one space or two spaces and one line, regardless of any accidentals.