"Preferred lies" is not codified in any of the 34 rules that make up the Rules of Golf. The practice is only mentioned in an appendix to the rules, where there is also a sample local rule for use by courses, clubs or committees.
Generally, preferred lies are put into place when adverse weather conditions (such as harsh winters) impact the golf course itself. Under the most common usage of preferred lies, a player whose ball is in the fairway would be allowed to improve the lie if that ball was, for example, on a bare patch of ground (where the turf had died due to weather conditions).
Preferred lies can differ from course to course; only a course, club or committee can declare preferred lies in effect (a player may not declare, in absence of a ruling from the club or course, that he is playing under preferred lies). If you carry a handicap, rounds played under preferred lies must be reported for handicap purposes.
To learn more about preferred lies, read our article in the Golf Rules FAQ: "What are winter rules (preferred lies)?"
