The transition level is the lowest available Flight Level for use.
The transition altitude is the altitude at which you can set the altimeter to 1013hPa so that it indicates flight levels.
The transition layer is the area between the transition altitude and transition level.
Semi-circular rule
Aircraft flying to the East (track 0°-179°) fly an odd level, aircraft flying to the West (track 180°-359°) fly an even level.
An even flight level is when where the last number before the final 0 is even e.g. FL80, FL100, FL 180.
An odd flight level is when where the last number before the final 0 is odd e.g. FL70, FL110, FL 170.
E.g an IFR flight that is operating on a track of 090° could use a cruising level of FL350.
VFR flights operating above the transition level are required to fly at flight level corresponding to the semi-circular rule described above plus 500 ft.
E.g. a VFR aircraft that is flying on a track of 090° could use a cruising altitude of 3500ft.
