Quadrantal rule (UK):
Levels to be flown by flights OUTSIDE Controlled airspace above 3000ft - mandatory for IFR flights, recommended ONLY for VFR flight. Level chosen is based on Magnetic TRACK, not heading.
(Flight Levels are what you get when you set 1013 on your altimeter).
000-089 - Odd FL's (FL30 (if available), FL50, FL70, FL90 etc)
090-179 - Odd FL's + 500ft (FL35, FL55, FL75, FL95 etc)
180-269 - Even FL's (FL40, FL60, FL80, FL100 etc)
270-359 - Even FL's + 500ft (FL25 (if Available), FL45, FL65, FL85 etc)
This is only applicable below FL195 in the UK.
The only time the above are not mandatory are when operating under the instructions of an ATC unit who may want you to fly at a different level.
Semi Circular rule:
ICAO standard (apart from Italy who have an interesting variation, but that may be for VFR only!) ensuring 1000ft separation between crossing tracks. Again - it's based on magnetic TRACK, not heading.
000-179 - Odd's (FL30, 50, 70 etc)
180-359 - Even's (FL40, FL60, FL80 etc)
This changes above FL290 in non-RVSM airspace (Reduced Vertical Separation Minima) but you don't need that for the PPL.