(a) *In controlled airspace.* Each person operating an aircraft
under IFR in level cruising flight in controlled airspace shall maintain
the altitude or flight level assigned that aircraft by ATC. However, if
the ATC clearance assigns "VFR conditions on-top," that person shall
maintain an altitude or flight level as prescribed by §91.159.

(b) *In uncontrolled airspace.* Except while in a holding pattern
of 2 minutes or less or while turning, each person operating an aircraft
under IFR in level cruising flight in uncontrolled airspace shall maintain
an appropriate altitude as follows:

(1) When operating below 18,000 feet MSL and --

(i) On a magnetic course of zero degrees through 179 degrees, any odd
thousand foot MSL altitude (such as 3,000, 5,000, or 7,000); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any even
thousand foot MSL altitude (such as 2,000, 4,000, or 6,000).

(2) When operating at or above 18,000 feet MSL but below flight level
290, and --

(i) On a magnetic course of zero degrees through 179 degrees, any odd
flight level (such as 190, 210, or 230); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any even
flight level (such as 180, 200, or 220).

(3) When operating at flight level 290 and above, and --

(i) On a magnetic course of zero degrees through 179 degrees, any
flight level, at 4,000-foot intervals, beginning at and including flight
level 290 (such as flight level 290, 330, or 370); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any
flight level, at 4,000-foot intervals, beginning at and including flight
level 310 (such as flight level 310, 350, or 390).