What are mongolian spots and where?

Mongolian patch: Mongolian spots or patches are areas in the skin where pigment producing cells make excess pigment. Usually confined to the back. Often in the area of the gluteal cleft in sizes that vary from small to large enough to cove the buttocks. Patches are occasionally seen on the upper back or backside of the arms. They have a purplish hue & can be mistaken for bruises.

See below:: Mongolian spots are very common in any part of the body of dark-skinned babies. They are flat, gray-blue in color (almost looking like a bruise), and can be small or large. They are caused by some pigment that didn't make it to the top layer when baby's skin was being formed. They are harmless and usually fade away by school age. They are commonly located on trunk/extremities.