One’s first cousin once removed is their mother’s cousin. A once removed relationship is the same relationship but separated by one generation. The child of one’s cousin is the individual’s second cousin once removed. This word is commonly confused with “second cousins.”
Second cousins are the offspring of a person’s first cousins. At least one great-grandparent is shared by second cousins. With first cousins twice removed, the great-grandparent of the younger generation is the grandparent of the older generation.
Although uncommon, there is a relationship known as double first cousins in which two sets of brothers marry and produce offspring with two sets of sisters. The children are double first cousins and share grandparents from both sets of grandparents.