Like language, we hear and read music in a linear fashion. We comprehend all but insignificant portions of such linear information only if it is organized into digestible amounts that our minds can process. Bars or Measures are among the symbols that organize music information into uniform segments of time – somewhat like periods, commas, etc. in language – so it is visibly and audibly comprehensible. In notated music Bars are indicated by a simple vertical line running through the lines of the staff. Typically, one bar is the smallest temporal unit containing the meter – often incorrectly called “beat” – of the song.
In this little example if one taps along with each syllable of “Yan-kee Doo-dle” one taps four times before arriving at the first Bar line. These four taps, or Beats, comprise one bar (the first) of this song. All the Bars of the song that follow also will have the same number of beats. Even at the last measure of this example, on the word “pony”, while there are only two pitches and two syllables to sing, we will still tap four beats, and the measure will last exactly as long as the measures in which we sing more than two pitches and syllables. The terms Bar and Measure are used almost completely interchangeably. “Bar” is, perhaps, slightly more colloquial than “Measure”, and non-musicians would more likely refer, e.g., to a “four-bar phrase” rather than to a “four-measure phrase”. One finds these terms in music copyright infringement opinions, typically to identify common portions of songs (e.g., Dorchester v. NBC, 1959) and to support determinations of infringement (the greater number of bars in common between two songs, the greater the likelihood of a finding of infringement.