The leg side, or on side, is defined to be a particular half of the field used to play the sport of cricket.
A cricket field is notionally divided into two halves, along a line running down the long axis of the pitch. In normal batting stance, the striking batsman stands side on to the bowler. The leg side is the half of the field behind the batsman. The half of the field in front of him is called the off side.
Note that the definition is relative to the batsman. If the batsman were to directly face the bowler, the leg side would be:
on the left side for a right-handed batsman, but
on the right side for a left-handed batsman.
This contrasts with terminology in baseball, in which left field is always to the left of the batter, while right field is always to the right.
The leg side is usually less well defended with fielders than the off side, because of the typical line of attack of the bowlers, which is frequently on or outside off stump. This makes it more difficult to hit the ball to the leg side because it involves swinging the bat across the line of the ball, which can lead to mishits and catches.
In recorded music, the terms A-side and B-side refer to the two sides of 7 inch vinyl records on which singles were released beginning in the 1950s.
The terms have come to refer to the types of song conventionally placed on each side of the record, with the A-side being the featured song (the one that the record producer hopes will receive radio airplay and become a "hit"), while the B-side, or "flipside," is secondary.
Since both sides of a single received equal royalties, some composers deliberately arranged for their songs to be used as the B-sides of singles by popular artists, thereby making a fortune literally off the back of the A-side.