Guido of Arezzo
In the 12th century, a development in teaching and learning music in a more efficient manner had arisen. Guido of Arezzo's alleged development of the Guidonian hand, more than a hundred years after his death, allowed for musicians to label a specific joint or fingertip with the gamut (also referred to as the hexachord in the modern era). Using specific joints of the hand and fingertips transformed the way one would learn and memorize solmization syllables. Not only did the Guidonian hand become a standard use in preparing music in the 12th century, its popularity grew more widespread well into the 17th and 18th century. The knowledge and use of the Guidonian hand would allow a musician to simply transpose, identify intervals, and aid in use of notation and the creation of new music. Musicians were able to sing and memorize longer sections of music and counterpoint during performances and the amount of time spent diminished dramatically.
Backhand
For most of the 20th century, the backhand was hit with one hand using either an "eastern" or "continental" grip. The first notable players to use a two-handed backhand were the 1930s Australians Vivian McGrath and John Bromwich. Beginning with Mike Belkin who was the first two-handed backhand player in the United States and Chris Evert in the 1960s, many players began to use a two-handed grip for the backhand. Pete Sampras and Stefan Edberg notably switched from the two-handed to the one-handed backhand late in their development.Generally, both backhands are efficient at what they do. Perhaps the biggest weakness cited by most coaches in regards to the one hander is the fact that it requires more time and steps to be set up properly - it requires an extra half step in order to execute a proper back-swing and have proper control over the point of contact (a one handed backhand is hit from a closed stance with the dominant foot placed in front, whereas the two hander can be hit both from open and closed stance). This hampers players who use a one handed backhand on fast surfaces as the high speed of most drive shots gives them very little time for preparation and setting up their shot. There are however plenty of players who can set up their shots properly even with little time for preparation and can hit effective backhands with one hand even on fast surfaces (the most notable example is Roger Federer who can hit drives with his backhand on all surfaces with the same consistency as two handers). Stan Wawrinka is another example of a player who hits consistent one hand backhand drives on all types of surface with the same margin of error. The type of backhand a player uses comes down to mostly personal preference and their game style. Since the 1970s however, the two-handed backhand has had a spike in popularity and is now more widely taught than the one-handed backhand.