NASCAR introduced the Chase for the Cup playoff format in 2004. The system was announced in January 2004 as the Chase for the Championship and first used during that year's NASCAR Cup Series season, then known as the Nextel Cup.
Before the Chase, the champion was determined by the full-season points standings. The new format kept the first 26 races as a regular season, then reset the championship contest for the final 10 races. Initially, the top 10 drivers—and any others within 400 points of the leader—qualified for the Chase.
The format was designed to keep more drivers mathematically involved late in the year and create a stronger championship focus during the final races. It was modified in 2007, changed substantially again in 2011, and expanded to 16 qualifying drivers under later versions.
NASCAR rebranded the Chase as the playoffs in 2017. That later terminology can cause confusion, but the original Cup Series playoff format began in 2004. The 2005, 2006, and 2007 seasons used an already established system rather than marking its introduction.