These extremely popular singers are called by some critics jazz singers and by others swinging pop singers, but not jazz. The issue is whether they improvise or not, which the purists say is mandatory to be considered a jazz singer. Another group of critics says the ability to swing is the most defining trait of jazz. These five singers swing but don’t improvise (as defined by jazz critics) and thus are labeled as jazz singers by some but not by others. “What difference does it make?” you may ask. None, really. Liking to listen to them is what counts. These five singers have each sold millions of records drawing from The Great American Songbook. They are frequently named as the best vocalists in American popular music.