If I had to pick a single feature to change how a face reads, I would pick the eyebrows. They take up almost no surface area and yet they steer the gaze of whoever looks at you, they set the resting expression and they change the apparent height of the forehead. A couple of millimetres of difference at the peak show. A haircut takes months to grow back, but so does a badly plucked brow, so it pays to go slowly.
What usually gets said about eyebrows stops at the arch. Round arch, angular arch, straight brow. The arch matters, but it is not usually what decides the result.
What does an eyebrow actually do to a face?
Three things, and it helps to separate them because they are adjusted separately.
It introduces a direction. A straight brow adds a horizontal. A high arch adds a vertical, because it pushes the gaze upward. An angled brow adds a diagonal.
It sets the height of the forehead. The higher you place it, the more forehead shows. The lower, the less.
And it fixes the resting expression, which is the face you make when you are not making any face at all. That is where most of this is decided, because it is the one people see ninety per cent of the time.
Why does face shape change what suits the brow?
Because the eyebrow is a line, and a line is always read in relation to what surrounds it.
The principle, if you keep only one, is this: a face welcomes directions it does not already have. A face built out of curves tends to welcome a diagonal. A face built out of angles tends to welcome a curve. A face that is already long rarely needs anyone adding height to it.
It is not an iron rule, and most of the time it allows for nuance. But it explains why the same arch that looks wonderful on a friend hardens your face.
If your face is round
The round face has similar length and width, with soft transitions and no marked angles. What tends to work here is an arch with a defined high point, because it introduces a diagonal where everything else is curve, and that diagonal lengthens visually.
The tail is better finishing outward and slightly down, rather than cut off abruptly. A very short tail tends to round things further.
If your face is square
Here the jaw is wide and angular, and the forehead goes along with it. The brow that usually accompanies best is a soft curve with some structure: not entirely straight, because it would repeat the horizontal of the jaw, and not a sharp angle either, because it would add one more angle to a face that already has several.
A soft arch, fairly full, with an extended tail, is what softens without erasing the character in most cases.
If your face is rectangular or long
This is where generic advice fails most. The long face already has height to spare, so a high arch usually lengthens it further.
What tends to balance is a flatter brow, with a low arch and the tail extended outward. A straight brow here does the same thing a fringe does: it cuts the vertical and gives width back.
If your face is heart or diamond
In both, the upper part is the protagonist, with a wide forehead on the heart and wide cheekbones on the diamond, and the chin narrows.
Here the brow tends to work better when it accompanies rather than competes: a soft curve, no hard peak, medium thickness. A very high, very marked arch tends to push the visual weight of the face even higher, which is the opposite of what we are after.
If your face is triangle
The jaw leads and the forehead is narrower. A brow with some presence, slightly fuller and with the tail well extended, tends to give width back to the upper part and distribute the weight better.
If your face is oval
The oval takes almost anything, which is precisely what makes it difficult. When there is nothing to compensate for, the brow stops being a geometry problem and becomes a decision about character: flatter reads calmer, more arched reads more expressive. Neither is wrong.
The detail that changes the result most
If you can only adjust one thing, adjust where the highest point falls. The most used reference places it between the pupil and the outer edge of the iris, looking straight ahead.
Moving it outward tends to lengthen the eye area and open the expression. Moving it inward usually hardens it considerably, and it is the most common mistake when someone plucks at home trying to "lift" the brow. It lifts, yes, but in the wrong place.
Something worth saying
Almost all of the above are tendencies, not laws. Face shape orients the direction of the brow, but thickness depends more on the size of your features and the contrast of your face, and the expression you want to project is a decision of yours that no proportion can make for you.
What I do hold to is the order: first know what your structure does, then decide. The other way round is copying, and copying is what produces those eyebrows that are technically well done and still do not quite fit the face.
Start by knowing your shape
The face diagnosis gives it to you by measuring, not by comparing yourself to drawings. You fill it in on screen and download it.
And the guide for your shape covers what suits it and what works against it across cuts, fringes, hairstyles, hair accessories, earrings and necklaces: the seven guides.