Despite the name, singles actually feature two songs. This is, of course, because a vinyl pressing has two sides - to release a vinyl single would necessarily mean releasing one of those two sides blank - so it makes sense to include two songs on a single. With those two songs, singles tend to work best when there is a clear hierarchy - where one song is clearly superior to the other - and this notion is easily reinforced by the two sides of the vinyl disc: one side is the "A side", featuring the superior song, while the other is the "B side", featuring a weaker song.
But the Beatles instead opted for a single in which the two sides were equal, thus the release of the "double-A-side single". Martin always regretted the decision of a double-A-side single. "It was the biggest mistake of my professional life. ... If I had stopped to think for more than about a second, I would have realized that one great title would fight another; and this is exactly what happened. The reports came in, and they showed that our double-A-side was selling extremely well. There was only one problem. The weekly sales figures showed that two singles, 'Strawberry Fields Forever' and 'Penny Lane', were selling well. They were being counted separately! As far as the charts were concerned, one side was effectively canceling out the success of the other. I firmly believe that if the total sales of those two sides had been added together we would have squashed the opposition flat" (Martin, page 26). Instead, it broke the Beatles' string of 12 consecutive number one hits, beaten out by Englebert Humperdinck's "Release Me".
Martin, George with William Pearson. With a Little Help from My Friends: The Making of Sgt. Pepper. Little Brown and Company, Boston, MA, 1994.