Personally id never use a regular ladder - it needs to be flat against the structure your climbing up to - i prefer A ladders - the rubber block is soft enough not to damage buildings - have pointed out to many a customer where they used normal ladder window cleaners that their sills are damaged or the brickwork/plaster is damaged from the old window cleaner.
With the A frame you can put the ladder at different angles around the window frames brickwork. Or place it in the centre of the frames say on a angled bay window..
Trying to explain how good A frames are for general window cleaning do them no favours, you really have to try them to realise how good they are.
I read all through this thread before I found someone with the right idea.
'A' frame ladders (more properly called 'tree bottomed points') were devised specifically for window cleaning by the ladder manufacturing industry and experienced window cleaners.
They were originally made by a company called Shaftesbury Ladders (unfortuneately long defunct)
The idea is based in simple geometry. Any three points will always lie in the same plane. That means that when you lean a 'pointer' against a wall, both feet and the top will all be in contact with the ground/wall.
That is not true of a parallel (open ender) ladder.
In spite of the H&S's blinkered and ignorant opinion, these are by far the safest ladders to use
IF YOU KNOW HOW TO USE THEM Unfortuneately the number of people still around who were trained by real window cleaning ladder experts is dwindling rapidly.
Accidents are increasing because untrained people are using inappropriate ladders in unsuitable applications.