Reading pitches from a staff with 11 lines can be confusing. Also, apart from the keyboard, the music notation for most instruments does not require all 11 lines of the Great staff.
Music is therefore written on staffs of five lines only.
But which five lines of the Great staff are selected ?
To identify the five lines a Clef is displayed at the beginning of the staff (Clef = 'key' in French).
In our present notation system two Clefs are most commonly used :