Inbound aircraft to London Heathrow Airport typically follow one of a number of Standard Arrival Routes (STARs). The STARs each terminate at one of four different RNAV waypoints (co-located with VOR navigational aids), and these also define four "stacks" where aircraft can be held, if necessary, until they are cleared to begin their approach to land. Stacks are sections of airspace where inbound aircraft will normally use the pattern closest to their arrival route. They can be visualised as invisible helter skelters in the sky. Each stack descends in 1000 ft (300 m) intervals from 16,000 ft (4,000m) down to 8000 ft (2,100m). If these holds become full, aircraft are held at more distant points before being cleared onward to one of the four main holds.