There are two main techniques controllers use to organize flight progress strips in order to best detect conflicts: grouped together by "altitude", or grouped together by route intersection points ("fix posting area"). Altitude grouping is the easiest and most common method when most aircraft in the sector tend to be level at a cruising altitude, such as in trans-oceanic sectors. Each aircraft's strip is placed in a "bay" (labeled section of the strip rack) depending on its altitude. Because aircraft cruising at different altitudes will never conflict regardless of route of flight or estimated times, they don't need to be checked against each other. Only aircraft at the same altitude (i.e. in the same bay) need to be further evaluated. If an aircraft needs to transition to a new altitude (whether changing cruising level, or descending to or climbing from an airport), it only needs to be checked against others in the bays between the current altitude and destination altitude.