Johann Sebastian Bach is one of the most important composers of all time, with his Baroque melodies enduring hundreds of years and still as magnificent as ever.
He wrote many pieces for piano, lutes and strings. This is the latter, written for a string quartet.
Strictly speaking, the most popular version is an arrangement by August Wilhelmj, who moved the piece down in pitch.
The song is a very moving one, alternating the shorter melodies of the violins with a longer, more drawn out finish. The way the instruments glide around each other is so masterful.
The piece is the second movement of the Overture number 3 in D major, MWV 1068, part of a suite of 4 overtures composed in the French style. An “Air” is a particular kind of instrumental song related to an aria.