From the 7th Century a system of Neums was introduced for the notation of plainsong. The Neums consisted of grave ( ` ) and acute ( ´ ) accents and horizontal lines, placed over the lyrics to indicate where the melody line was going up or down.
The sign of the trill in our present system is a relic of the old Neum system.
To present accurate note pitches the device of the staff (or stave) was added.
This began as a single horizontal line, then more where added until a four line staff was standardised for the plainsong (= simple melody line without chords or harmony). This staff was probably introduced for the first time by Guido d'Arrezo, a famous music expert around 1000-1050.
The notes were written on (and in between) the lines of this staff which defined there exact pitch.
In the late 13th Century a classification of note head shapes also defined the note duration. This important step is ascribed to Franco of Cologne.