Complex sentence: Rugby sevens-a short form of the sport of rugby union-was first played in 1883, with the first (men's) internationals taking place in 1973.
Split into simpler sentences: Rugby sevens--also known as 7-a-side, or 7s--is a short form of the sport of rugby union that was first played in 1883. The first (men's) internationals took place in 1973.