Vertical lines lengthen and can slim the body. They create the illusion of height or length. The eye moves quickly over them.
Horizontal lines broaden or widen. The eye moves more slowly over them and we are constantly looking for horizontals as we search for the horizon as it gives us a feeling of balance. Every hem (unless it’s asymmetric) is a horizontal line.
Diagonal lines move the eye and create action. The more ‘vertical’ the diagonal the more it works to slim, the more ‘horizontal’ the diagonal it becomes more broadening. In Western cultures where we read left to right, we also look at horizontals from left to right, so some take our eye upwards, others downwards.
Curved lines add volume (or as my American image consultant friends say “round adds pounds”). They are the slowest lines for our eyes to follow as we have to move over more area to follow them (this is why they add that volume).
Understanding a little about lines helps us think about how to dress our body shape, how to alter it using our ‘magician’s toolbox of illusion’. Want to look taller? Add in some more verticals. Want to balance your hips? Add a horizontal line element across your shoulders. Want to move the eye upwards, consider wearing a jacket which draws the eye upwards (vertically with it’s opening) and then outwards (with it’s diagonal based lapel). Want to add curves? Put a frill over that area.