40 (forty) is the number that follows 39 and precedes 41. Though it’s related to the number “four” (4), the modern spelling of 40 is “forty.” The older form, “fourty,” is treated as a misspelling today. The modern spelling could reflect a historical pronunciation change.
If you catch yourself misspelling the name of this number as fourty, you’re not alone. It’s a common mistake, both in print and online: