THEORY:The eighth notes (straight by definition) divide time into two equal parts, i.e. you play two notes, of EQUAL duration, per beat.The shuffle eighth notes divide time into three equal parts. The first note occupies the first two thirds and the second note occupies the last third. See figure I.
Here are two popular songs. One uses straight eighth notes (Are you sleeping brother John?), wheras the other uses shuffle eighth notes (My Bonnie is over the ocean.). See figure II.