An account in the names of two people as “joint tenants” carries with it a “right of survivorship.” This means that if A and B are joint tenants, and B dies, A gets the account (all of it). This result occurs even if A has a will that gives all his/her property to C. The Act forces this result, in spite of the words of the will. There is a small exception to this if B has put A on the account only as a “convenience” (i.e. A only meant to give B the ability to write checks). However, the law presumes that joint tenant accounts are not convenience accounts, and C would have a difficult time proving otherwise.
A second way for people to co-hold accounts is as “tenants in common,” or, alternatively “joint account, no survivorship” In this case A and B are tenants in common, and when B dies, her will does govern the disposition of her account; and her share goes to C, the person named in her will.
Finally, accounts held by husband and wife, as “husband and wife” or “by the entireties” pass to the surviving spouse. Again, the deceased spouse’s will does not govern who owns the account.
— Rod Fluck