more letters followed by one or more digits (e.g., V484 or UA419). The airway structure is divided into high and low altitudes. The low altitude airways in the U.S. which can be navigated using VOR Navaids have names that start with the letter V, and are therefore called Victor Airways. They cover altitudes from approximately 1200 feet above ground level (AGL) to above mean sea level (MSL). T routes are low altitude RNAV only routes which may or may not utilize VOR NAVAIDS. The high altitude airways in the U.S. have names that start with the letter J and are called Jet Routes, or Q for Q routes. Q routes in the U.S. are RNAV only high altitude airways, whereas J routes use VOR NAVAID's the same way V routes do. J & Q routes run from to . The altitude separating the low and high airway structures varies from country to country. For example, it is in Switzerland, and in Egypt. Navaid Navaid routing occurs between Navaids (short for Navigational Aids, see VOR) which are not always connected by airways. Navaid routing is typically only allowed in the continental U.S. If a flight plan specifies Navaid routing between two Navaids which are connected via an airway, the rules for that particular airway must be followed as if the aircraft was flying Airway routing between those two Navaids. Allowable altitudes are covered in Flight Levels. Direct Direct routing occurs when one or both of the route segment endpoints are at a latitude/longitude which is not located at a Navaid. Some flight planning organizations specify that checkpoints generated for a Direct route be a limited distance apart, or limited by time to fly between the checkpoints (i.e. direct checkpoints could be farther apart for a fast aircraft than for a slow one). SIDs and STARs SIDs and STARs are procedures and checkpoints used to enter and leave the airway system by aircraft operating on IFR flight plans. There is a defined transition point at which an airway and a SID or STAR intersect. A SID, or Standard Instrument Departure, defines a pathway out of an airport and onto the airway structure. A SID is sometimes called a Departure Procedure (DP). SIDs are unique to the associated airport. A STAR, or Standard Terminal Arrival Route, ('Standard Instrument Arrival' in the UK) defines a pathway into an airport from the airway structure. STARs