You can (theoretically) read a score without bar lines, and indeed bars (or measures) and bar lines, in the sense that we use today, are a relatively new invention, from the mid of 17th century. Before that, "bar" lines were not used at all, or were used only to visually divide a piece into sections or phrases. In fact if we go all the way back to Gregorian chant, even the concept of beat, or regular time counting, is somewhat vague or even non existing.
But as European erudite music evolves, a number of things happen:
- Stylistically, the beat or pulse of the music becomes more important and a distinctive integral part of each particular musical style. The rhythmic division (binary and ternary) and the existence strong and weak beats, important in popular music such as dances and work songs, is also incorporated in erudite music.
- Musical composition becomes more complex, with multiple instruments/singers performing different parts and polyphony (two or more simultaneous lines of independent melodic parts) becomes more and more important.
And so it became necessary to have a notation device that allowed for the correct understanding by the performers of the intended rhythm and synchronization of the different performers.
Today the concepts of measure and time signature and the bar notation are fundamental parts of western music, that makes possible in a very practical way, for example:
- To identify a binary meter, like a march, or ternary meter, like a
- To identify at a glance the strong and weak beats of each measure.
- To notate extremely complex rhythmic figures (including polyphonically), with irregular subdivisions, while at the same time maintaining a clear perception of the beat.
- To precisely notate pieces with complex signature (5/4, 7/8, etc.).
- For an orchestra or ensemble to rehearse (measure numbering is a fundamental part of instrument-parts scores).