triple X syndrome

Worth naming plainly here too, since it often surprises people who assume biology itself draws a simple clean line, it doesn't.

Documented natural chromosome variations include XXY, known as Klinefelter syndrome, typically presenting as male, XYY, sometimes called Jacob syndrome, also typically male, XXX, sometimes called the triple X syndrome, typically female, and XO, a single X chromosome with no second sex chromosome at all, known as Turner syndrome, also typically female.

Some people carry even rarer combinations.

And beyond chromosome count itself, there are cases where the SRY gene, the specific gene on the Y chromosome that actually triggers male development, get damaged, producing someone with XY chromosomes who develops as female, or copied over onto an X chromosome instead, producing someone with XX chromosomes who develops as male.