Shape is a big aspect, and it can be difficult to figure out because we all need a different shape. The shape that looks best on me may look worst on you. It all depends on our individual features and the look we're trying to achieve (sexy, innocent, natural, etc). In my opinion, however, there are a few universally bad shapes...
Straight as a board
Spacing is another major thing to consider. The general guidelines are that the inside end of your eyebrow should be aligned with the inside corner of your eye and/or the edge of your nostril. A lot of people make their eyebrows too short, and others make them too long, including leaving no gap between the two at all...
Way too large of a gap
Too close together
No gap at all
Next on the list would be thickness. A thinner eyebrow tends to make a person look older, while a thicker eyebrow makes a person look younger. Children have thicker eyebrows naturally, and so the look tends to give off a look of youth and innocence. As we age, our eyebrows often thin out naturally. Plus, thin eyebrows were the look decades ago, and a lot of older women haven't let go of that, so our minds are kind of trained to associate thin eyebrows with an older face. If you're specifically trying to make yourself look younger or older, you can adjust the thickness of your eyebrows accordingly...
Way too thick
Way too thin
The last, but still incredibly important, thing to consider is color. This is one of my biggest beauty pet peeves. I see so many people change the color of the hair on their head and then totally neglect their eyebrows. Folks will bleach their hair and have dark brown or black eyebrows. They'll dye their hair a dark color and leave their naturally blonde eyebrows. Bleh! A basic rule of thumb is that if you have blonde hair, your eyebrows should be 1-2 shades darker than your hair and if you have dark hair, they should be 1-2 shades lighter. I tend to favor a more perfect match when I have dark or red hair, but a nice taupe when I'm blonde.
Light hair, dark brows
Dark hair, light eyebrows
There are a few celebrities who always have perfect eyebrows and serve as a great inspiration.
My suggestion to you is to take yourself to a Benefit Brow Bar or a waxing salon in your area and have your eyebrows professional shaped and waxed. Once you've gotten help choosing the perfect shape for you, and have gotten them into that shape, the maintenance will be easy for you - just tweeze any strays that pop up. But in my opinion it is highly worth it to get help finding the perfect shape and thickness for you. If your eyebrows are currently too light, go ahead and fill them in with a darker powder or pencil, or even dye them. If your eyebrows are too dark, bleach them. YOU get to pick the color. Darken them to your liking or lighten them to your liking.
All that I've said above is my opinion. You may have eyebrows that I've described as "bad" but you may love them. That's okay. There are plenty of people who don't like my eyebrows. I consider the above references to be generally bad concepts but that doesn't mean that every single case of those is always going to be bad or that every single person will agree with me. As long as you're happy with what you've got, that is all that matters.
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