Sheet music, sometimes called "score," is the hand-written or printed form of musical notation, or the system that represents aurally perceived music via the use of written symbols. The history of musical notation, and therefore of sheet music, is a long one. There are those who hold that the earliest sheet music known is a fragment of a cuneiform tablet from Nippur, an ancient Babylonian city, and dates from approximately 2000 BCE. While this music was written on a clay tablet rather than paper, it still constitutes sheet music because the term "sheet" merely differentiates music on paper from audio presentation. So although the tablet's notations are fragmentary and somewhat simple, it is probably safe to say that they represent the earliest recorded melodies in the world.
Anyone familiar with ancient Greek civilization will not be surprised to learn that the sheet music of those people was relatively complex. Ancient Greek musical notation was capable of representing pitch, note duration and, to a limited extent, harmony. It consisted of symbols placed above text syllables and was in use from at least the 6th century BCE until approximately the 4th century CE, a date that coincides with the fall of the Roman Empire.
Sheet music, like Europe as a whole, suffered a major blow when Rome fell. The art of writing music all but vanished during the times that followed, which are commonly referred to as the Dark Ages. However, by the middle of the 9th century, musical notation began to revive thanks to the Roman Catholic Church. The Gregorian chant was a ubiquitous form of worship in those days, and the monks performing it developed specific symbols, neumes, in order to record it on paper.
It is to another style of religious song of this period, the plainchant, that we owe our modern form of sheet music. The original system of writing plainchant did not utilize a staff. Although capable of expressing considerable musical complexity, such a system could not convey exact pitch or time. Sheet music from this genre served mainly as a reminder to a performer already familiar with the tune rather than a means by which a novice might learn a new song. To deal with the problem of exact pitch, a staff was introduced, originally consisting of a single horizontal line but eventually comprised of four parallel horizontal lines, which became the standard. The vertical position of a mark on the staff indicated the pitch at which it was to be sung or played. Anyone who looks at music from this period will easily recognize the roots of modern sheet music.
From the late Dark Ages until the 15th century, western sheet music was written by hand and generally preserved in large, bound volumes of manuscripts. The best known examples of such manuscripts are those of the previously mentioned plainchant, which is a form of monophonic chant. The advent of the printing press in the middle of the 15th century, of course, irrevocably changed how sheet music was created. However, it took several hundred years for printed sheet music to become the norm, and much music continued to exist solely in hand-written manuscripts until well into the 18th century.