It's interesting to see how Steely Dan have morphed over the years, and how their sound, as a whole, changed during their lengthy hiatus.  Kicking off in 1972 with "Can't Buy A Thrill", the sound was predominantly guitar-driven, with hints of jazz chops and addictive hooks.  By the time "Pretzel Logic" hit the shelves in 1974, vocalist Michael McDonald had enhanced the Dan's musical schema with some blue-eyed soul, and the hits started pouring out.  "Rikki Don't Lose That Number".  "Black Friday".  The title track--all classics.  With "Katy Lied" and "The Royal Scam", SD anchored themselves into the mainstream-cum-progressive category by continuing to push the envelope with avant-garde chord changes and airtight harmonies.  So, when "Aja" debuted in 1977, the Dan were ready to drop a bomb into the spotlight.  A tough act to follow, "Aja" raised the bar for the wannabe acts with masterful musicianship by Wayne Shorter, Jeff Porcaro, Larry Carlton, McDonald (on his fourth album with the Dan), and heightened writing and performances by Becker and Fagen.