It was in the 1950s that record companies spotted an opportunity to release a new format that occupied a space between singles and long-playing records. Whereas singles were traditionally two tracks, and albums anything from 8 to 12 tracks, EPs (so-named to denoted "extended play") had four tracks. Artists often released these as “bite-sized” album-tasters, and in some cases, they even released all the tracks from an LP on a series of EPs, as they did with The Beatles' Yesterday EP.
Parlophone, The Beatles UK label had released a whole string of EPs by the band starting with “Twist and Shout” in the summer of 1963; it naturally topped the EP chart, a feat equaled by six more of their EP releases and when their Yesterday EP came out on March 4, 1966, it quickly climbed the chart and became their seventh No.1 on March 26, 1966, going on to spend seven weeks at the top of the bestsellers list.
Source:Richard Havers/yahoo.com