As the other day someone asked: Why do we notate keyboard sheet music as we do ... (the question has been probably put on hold as off-topic ...)
I think this question is interesting: Why do we notate the keyboard music in grand staff?
What has been before? Probably tablatures. To day lot of Popsongs and Jazz standards are published in fake books in 1 staff and with chords abbreviations.
Well, my question in this context is: When was the change from tabulatures to grand staff?
PhD musicologist here, so I'm just going to pontificate without giving sources.
Both staff and tablature go back to the very earliest keyboard notation we have, the "Robertsbridge codex" from about 1360. It uses staff notation for the right hand and tablature for the left. There's a facsimile in the Wiki article.
Each kind of notation has its own advantage. Tab is much more compact, so would be used when paper is expensive. I would also guess that it's quicker to copy. It was usually used in 17th cent. German organ music, for example, which was hand-copied, typically by students. Paper cost roughly the equivalent of $5 a sheet. A bit later, Bach usually wrote his organ pieces out in staff notation, but occasionally used organ tab when he ran out of room on a page.
Music printing from movable type came in just after 1500. Keyboard and lute/guitar (etc) music is difficult to print in staff notation because movable type pretty much limits you to one voice per staff. But some kinds of tab are easy to set in type. There are some big 16th-cent. Spanish publications of keyboard music in Spanish tab, for ex. In Germany, Scheidt solved this problem by printing his works in open score (one voice per part) and told his readers to copy everything out in tablature and play it in the usual way. (Since the "usual way" of circulating keyboard music was by copying it, this isn't quite as obtuse as it sounds.)
Keyboard music could be engraved, but this was extremely expensive, since it meant a lot of skilled handwork - copying the music in a scritch-scratch fashion backward on an engraving plate. Bach's published keyboard works, for example, cost the equivalent of hundreds a dollars per volume. The price of engraving dropped in the middle of the 18th century when the punch was invented, and this was the predominant technology until digital typesetting came along. The only reason to use tab from that point on would be for the convenience of the player - lute players to this day much prefer to play from tab, for ex.
The following article gives an answer to this question not only when but also by whom: It says it was about the time of the birth of Bach that German keyboard players adopted the system of a grand staff influenced by French publications.
I wonder where we would be today interpreting Bach and how Bach would have written his work if the full score and grand staff wouldn't have been developed at his time.
Notation
Only in the late seventeenth century, under the influence of French publications, did German keyboard players adopt the system of keyboard notation on pairs of five-line staves. Pedal parts were normally included on the lower staff, not on a separate one of their own as in modern organ scores. Some manuscripts, especially those containing Italian or English music, still employed the six- and eight-line staves used in those countries. In such manuscripts the placement of notes on the upper or lower staff is an indication to use the right or left hand, respectively; a melodic line exchanged between the two hands, as in the inner voices of many contrapuntal pieces, wanders between staves. But many German musicians continued to employ the older form of notation known as tablature, using letters instead of notes. This more abstract notation requires the player to determine which notes belong to which hand (or to the pedals). Although more economical—a composition could be copied using a half or a third as much paper—tablature is harder to read and by 1700 was going out of fashion. J. S. Bach continued to use it well into the eighteenth century, but only to notate brief sketches or when forced by lack of space to write a few measures of music in the margins of a page otherwise employing staff notation. One of the few substantial German publications of keyboard music from the first half of the century, the Tabulatura nova (Hamburg, 1624) of Samuel Scheidt (1587–1654), was printed in open score, using a separate staff for each contrapuntal line; Scheidt nevertheless expected players to transcribe this into tablature notation.
Edit!
Looking for more information I’ve found this source:
The treatise Musica enchiriadis (AD 900) uses Daseian notation for indicating specific pitches, but the modern use of staff lines is attributed to Guido d'Arezzo (AD 990–1050), whose four-line staff is still used (though without the red and yellow coloring he recommended) in Gregorian chant publications today. Five-line staves appeared in Italy in the 13th century and it was promoted by Ugolino da Forlì; staves with four, five, and six lines were used as late as 1600.
