Line, in any visual composition, exists to control what you look at and what you look away from. It also controls how quickly the eye travels over your outfit/composition.
Utilize style lines and attention getters (buckles, prints, buttons, or accessories) to draw the eye where you want it and move it briskly along from where you don’t. Note – your entire outfit, including your skin and hair, earrings, necklace, bracelets and shoes – these *all* are style lines or attention getters, not just the ones on your clothing! This is why old-school advice is to not wear ankle-strap shoes if you have heavy legs, not because you might not have excellent ankles, but because a horizontal line shortens/widens. (I wear ankle straps anyway, I have excellent ankles and straps are an attention getter).
Vertical lines move the eye up and down, horizontal lines move the eye from side to side, diagonals diagonally, curves… you get the idea, yes? The eye will follow the line until it finds somewhere interesting to stop.
Horizontal lines include your hemline, your neckline, and your waist seam. This is why it’s not a good idea to end your sleeves right at your bustline if you’re on the well-endowed side… or to make sure that they’re set an angle, so that the eye moves in a curve.
An excellent exercise is to take a picture (use that camera!) and see how many lines you can identify on your figure… this will help you see where you’d like to “speed up” the travelling eye and where you’d like it to pause. One usually wants to draw the eye in a triangle or circle around the face/neck. That’s why earrings are so essential – they encourage a circular visual pattern between eyes, lips and ears.
Even though seam lines are subtle, they still give the eye something to travel along, and thus they break up the space. Cut anything right in half, and you’ll make it look overall larger. (Skirts with a center seam look great on the wide-of-hip). Cut things off-center, and the eye shrinks the size of the object. (Or, why I wear six-gore skirts, with the seams on either side of my stomach – makes things look smaller).
Style lines affect the feel of an outfit. Lines that move you briskly from one place to the other give one the feeling of energy. Lines that curve around and get you around eventually give a more sensual effect. Fewer lines/seams? Less movement, a more formal/staid effect. Still water rather than a rushing stream. Your use of line and motion should reflect your personality as well as your event.
The more athletic your figure, the more natural you will look with energetic clothing that looks like it has motion. A quick google of Kate Middleton when she’s on duty vs. when she’s having fun will give you an excellent example.
The more rounded your figure, the more natural you’ll look in curved lines – this is why bias cut dresses and slips are so popular. They encourage the eye to drift slowly from hip to hip, clinging to the curves of a woman’s body. Yes, your eye can see the texture/weave of the fabric. YOU might not notice, but the eye will pick it up and it will affect the overall look.
Line is extremely important, and you should be aware of what lines are most likely to look well on you, and which do not. No matter what colors you wear, what fabrics you indulge in, your basic style lines of flattery won’t change.