In Western cultures, because we read left to right, the left to right diagonal moves the gaze upwards (towards the face).
The right to left diagonal moves the gaze downwards (towards the hips or ground) and has a more negative feel.
Now for the men in your life – his tie should also have an upwards (left to right) diagonal stripe to draw attention up to his face.
So how about V Necks – they are one of the most common diagonal lines found in clothing.
The more the V is vertical, the more it behaves like a vertical line, though direction may change depending on the width.
A medium diagonal will draw the eye upwards (such as a standard V neckline).
A narrow diagonal will draw the eye downwards as it becomes more like a funnel.
A wide diagonal will draw the eye outwards and behaves more like a horizontal lines and broadens (read more about horizontal lines here).
Halter necks and raglan sleeves (like the top left dress in the pic above) will narrow a broader shoulder.
The middle dress will broaden shoulders, but also, because of the second lower V shape, will make the rib cage and bust look larger.
If you want to add curves, look for diagonals that are narrower at the waist and wider below and above, like the third dress above.
A diagonal hem is a great way to avoid adding an extra horizontal line across a wider part of your body and to your outfit too!