These superstar artists are called jazz singers by some critics and swinging pop singers by other critics. 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. Appeal is what counts. These five singers have each sold millions of records drawing from The Great American Songbook.