Black triangles, or open gingival embrasures if you want the proper name, are the small dark gaps that appear between teeth just below the contact point. They are extremely common in adults, they are rarely a sign of anything dangerous, and they bother people far more than most dentists acknowledge.
Why they appear
Healthy gum sits as a small peak, the papilla, filling the triangle of space between two teeth. A black triangle is simply that peak no longer reaching the contact point. Several things cause it, and usually more than one is involved.
Gum recession after periodontal disease is the most common. Once the bone that supports the papilla has been lost, the gum has nothing to sit on. Age-related recession does the same more slowly. Orthodontic treatment can reveal them, because straightening rotated teeth changes where they touch. And overzealous brushing with a hard brush wears the gum back mechanically.
Tooth shape matters more than people realise
Triangular teeth, wide at the biting edge and narrow at the gum, leave a larger space to fill than square ones and are much more prone to black triangles even with perfectly healthy gums. If your teeth are that shape, this was always more likely, and it is not something you caused.
What actually works
Stabilise the gums first. If there is active periodontal disease, nothing cosmetic should happen until it is treated, or you will be rebuilding onto a moving foundation.
Composite bonding is the workhorse. Adding a thin amount of composite to the sides of the two teeth closes the gap from the sides rather than filling it, and it is done in one visit without drilling. It is the least invasive option that produces a genuine change, and it is reversible.
Orthodontics helps where the teeth are rotated or tipped, because bringing the roots parallel moves the contact point downwards and the papilla can fill the space again.
Gum grafting to rebuild a lost papilla is possible but unpredictable, and honest practices will tell you the success rate for that specific procedure is poor compared with grafting elsewhere in the mouth.
What usually is not the answer
Veneers or crowns are often suggested for black triangles, and they do close them, by making the teeth wider all the way to the gum. The problem is that this means preparing two healthy teeth to solve a gum problem, and the resulting teeth can look bulky at the neck. If bonding can achieve the same result without touching the underlying tooth, it is the better trade.
Living with them
They collect food, which is the practical complaint more often than the appearance. Interdental brushes sized to the gap clean them far better than floss does, and keeping them clean stops the recession progressing. Plenty of people, once told the gaps are stable and not a disease, decide to leave them alone entirely. That is a perfectly reasonable decision.
What bonding them actually involves
It is done without anaesthetic in most cases, because nothing is drilled. A thin matrix strip is placed between the teeth to shape the composite, material is added in layers against the side of each tooth, and each layer is cured with a light. The critical part is the finish at the gumline: composite left slightly proud there will inflame the gum and undo the point of the exercise, so the margin is polished carefully and flossed through before you leave.
A pair of teeth takes around an hour. Most people having black triangles closed are treating the two or four teeth that show when they smile rather than the whole arch.
Stopping them getting worse
Whatever caused the recession usually has not gone away. Switch to a soft brush and stop scrubbing horizontally, which is the single most common mechanical cause. Use interdental brushes rather than floss, sized so they fit snugly without forcing. Keep hygiene appointments, because inflamed gums recede faster than healthy ones.
If you grind, a night guard is worth discussing. Grinding loads the necks of the teeth and contributes to both recession and the wear that opens contacts up.