Refer to figures.

• Transition altitude → The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes using QNH.
• Transition level → The lowest flight level available for use above the transition altitude. At or above the transition level 1013.2 hPa pressure setting is used and the altimeter indicates flight levels.
• Transition layer → The airspace between the transition altitude and the transition level. When climbing through this layer 1013.2 hPa setting is used and the altimeter indicates flight levels, and when descending through this layer QNH is used and the altimeter indicates altitudes.

ICAO Doc 8168
2.1.4 References to vertical position

2.1.4.1 The vertical position of aircraft operating at or below the transition altitude shall be expressed in terms of altitude. Vertical position at or above the transition level shall be expressed in terms of flight levels. This terminology applies during:

a) climb;
b) en-route flight; and
c) approach and landing (except as provided for in 2.4.3, “References to vertical positioning after approach clearance”).

2.1.4.2 Passing through the transition layer. While passing through the transition layer, vertical position shall be expressed in terms of:

a) flight levels when climbing; and
b) altitude when descending.