Will publish a 371 page book about the subject.
With my airline, had to lecture 3 hours on the subject for new hires.
And they all at least had a CPL/IR...
So for our friend PompeyPaul (I do not consider him one of the dummies).
And thinking of the UK/Europe environment.
Never flew VFR in UK, so excuse my lack of knowledge.
Do not know if UK does quadrantal or semi-circular altitudes/levels in VFR...
xxx
At low altitude, around airports, you will normally fly with a QNH.
Some guys flying touch and go do it QFE... Good idea for circuits.
xxx
If you climb higher, often above 3,000 ft QNH, you are passing the transition altitude.
Reset your altimeter to 1013.2 hPa (= QNE)... you are now in flight levels.
You should climb to (at least) the transition level. It is your lowest possible cruising level.
You cannot cruise from A to B in the transition layer airspace.
Transition layer is only for climb or descent.
And its "thickness" varies with high or low altimeter settings.
xxx
After a cruise, you descent through the transition level, say FL45.
Reset your altimeter to the local QNH for low altitude, and approach/landing.
xxx
Altimeter vocabulary -
Say "altitude" when you fly with an altimeter on QNH.
Say "height" when altimeter is on QFE.
Say "level" when cruising with altimeter in QNE.
xxx
Do not confuse yourself with the USA or or other procedures for metric airspace...
There are inches and millimeters of hPa to study about...
1013.2 hPa (former mB) = 29.92 inches = 760 millimeters.
That is off your subject.
xxx
One number for you to remember is 28.5 feet.
Every 1 hPa error in QNH setting will give you that 28.5 ft altitude error.
Many pilots round-it off to 30 feet.
xxx
Happy contrails