10 south of west is a way measure of the direction between south and west?

Yes the direction pointing 10 degrees south of West is found by drawing a North/ South line and West /East line and starting with your pen on the direction pointing West and then moving south of this by 10 degrees on a protractor. i.e going south of the west pointing line! The centre of the protractor is placed centrally where the lines meet.The line from the centre to this place 10 degrees further south, we say points 10 degrees south of Westbecause it does! You can't start at south and move up towards west by 10 degrees though..that would be called something else, because it lands in a different place. Descriptions tend to say south of West not west of southby convention. We hear "North of East" not "East of North". Proximity to North or to South being of primary interest.