Moles. Beauty marks. Birthmarks.

No matter what you call them, they are just one of those inevitabilities in life—those mysterious scattered little, brown spots that almost all adults have. According to the American Academy of Dermatology (AAD), people with light skin tend to have more moles, but in general, adults typically have anywhere from 10 to 40 scattered across the body.

Moles typically appear during childhood and adolescence, though they may continue to show up later in life, and they can develop anywhere on your body — even your scalp, armpits, or under your nails! Because moles form in part due to their exposure to UV light, they tend to appear on parts of the body that aren’t covered up.

But why are they there? Simply put, moles are small, dark brown growths on the skin caused by clusters of pigmented cells, called melanocytes, according to Mayo Clinic. Melanocytes exist throughout your skin and produce the natural pigment melanin, but they can also grow in clumps, creating moles. (Technically, freckles are also melanocytes, although moles and freckles are different.)

READ MORE: I was Diagnosed With Melanoma (And It Sucked)

That said, there are multiple types of moles, and each has its own distinctive characteristics. The common mole is, well, the most common type to have — and is generally harmless — but it’s not the only type. Read on for info on a few other varieties, and what causes moles in the first place.

Atypical Moles (Dysplastic)

Dysplastic moles are usually hereditary and tend to be odd-shaped, larger than a pencil eraser, and a mix of colors (including tan, brown, red, and pink). They’re normally found on the back, but can appear anywhere on your body — just rarely on your face.

Though on their own, atypical moles aren’t indicative of melanoma, if you have four or more, the risk for getting the disease is higher, according to the AAD. The presence of many atypical moles could be a sign of Familial Atypical Multiple Mole-Melanoma (FAMMM) Syndrome, a rare hereditary syndrome. According to Johns Hopkins University, FAMMM is caused by inherited gene mutations.

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Congenital Moles

Congenital moles, also called congenital nevus, are ones that are present from birth. According to the AAD, roughly 1 in 100 people are born with a congenital mole. Certain prenatal issues, such as a growth of fatty tissue cells, Spina bifida (a birth defect in the spine), and Neurofibromatosis (an disease that involves changes in skin pigment) may potentially lead to the growth of congenital moles, according to the U.S. National Library of Medicine.

Though congenital moles often start out small, some grow throughout childhood. Large congenital moles (bigger than two inches in diameter) represent an increased risk of developing melanoma, and so are often removed by dermatologists.

Spitz Nevus

These pink, raised, and dome-shaped moles usually appear before the age of 20, according to the AAD. Though these moles are benign, their appearance can resemble melanoma even to the trained eye of a dermatologist—so don’t be surprised if your doctor chooses to remove or biopsy the mole to be on the safe side.  According to the American Osteopathic College of Dermatology (AOCD), the cause of these moles—which may bleed or itch at times—is currently unknown.

The important thing to remember is that if you are concerned about a mole, go to a dermatologist and get it checked out, stat! If a mole is changing color, increasing in size, or bleeding it could be a bad sign.

READ MORE: Holy Moly: What You Can Do Now to Prevent Moles Later