People gradually realized that it was necessary to find a system which showed the exact pitch of the note. The first stave was just one single horizontal line. Gradually this increased to four lines. Guido d’Arezzo, an Italian Benedictine monk who lived in the 11th century developed this system which soon led to the way of writing music that we know. By the 16th century everybody was using the 5 line stave. A system of bars (measures) was used everywhere by the end of the 17th century. At first the bar lines where there to show which notes should be played or sung together. Gradually the system we know today was developed in which each bar lasted the same length of time, and has a particular number of beats (counts) for each bar. By the end of the 17th century this was used by most musicians in European classical music.