What was NASCAR's gentleman's agreement?

Dale Earnhardt Jr. spins after a blown tire, causing a yellow flag during a NASCAR Sprint Cup Series race at Dover International Speedway in Dover, Del.­ See more NASCAR pictures.
AP Photo/Carolyn Kaster

A gentleman's agreement -- a contract with no need for forms, files or signatures, enforced solely by trusting in the honor of the other parties to the agreement -- seems like a strange way to govern a sporting event, doesn't it? After all, every professional sport relies on a written rulebook, right? But the world is full of unwritten rules, too, and even though they aren't codified in any book of laws, there are still consequences if you break them. The world of sports, which certainly has plenty of written rules, has its share of unofficial rules as well. Don't run up the score on a struggling opponent. Pass the ball to a teammate when he has the chance to break a record. Violate these rules, and the results won't be measured in points or penalty minutes, but your peers will make sure you feel them just the same. Angry words in the post-game press conference might be the least of your worries. Some transgressors spend months looking over their shoulder on the field, court or track just waiting for payback.

­­This brings us to NASCAR's gentleman's agreem­ent, a rule that governed the behavior of all the drivers in NASCAR's top racing series for almost 30 years, despite never being written into a single rulebook. What was the agreement, and why is it no longer in effect? Are there simply no gentlemen in racing anymore? Or did something else make it obsolete?