The document that says who inherits what - and, just as importantly, the many things it does not control.
A will is a written direction, taking effect only at death, saying who receives the property that passes through the deceased person's estate and who is to administer it. Until death it controls nothing and can be changed or revoked at any time by someone with capacity. It is not a transfer: nothing moves on signing, and the person keeps full ownership of everything named in it.
The most consequential thing about a will is how much it does not reach. Property that passes by its own mechanism is not controlled by the will at all, no matter what the will says. A life insurance policy, a retirement account or a payable-on-death bank account passes to the named beneficiary; property held in joint tenancy with right of survivorship passes to the surviving owner; property already placed in a trust passes under the trust. These are collectively the non-probate estate, and in many ordinary estates they are the larger half. A will that leaves "everything equally to my three children" while an old retirement account still names a former spouse does exactly what both documents say, and the account is not divided.
What makes a will valid is set by state statute and is formal rather than substantive: writing, signature, and witnesses who sign in the manner the state requires. Most states additionally allow a self-proving affidavit, a notarised statement signed at the same time that lets the court accept the will without locating the witnesses years later - a small step that removes a common practical difficulty. Some states recognise handwritten (holographic) wills on their own terms; some recognise none. A will typically also names an executor, may name a guardian for minor children, and may include a residuary clause catching anything not specifically given away - the omission of which is a routine source of unintended intestacy for part of an estate.
Two later-life problems account for much of the litigation about wills, and both are worth understanding while there is still time to prevent them. The first is where the original is: many states presume that a will last known to be in the maker's possession and not found after death was deliberately destroyed, so a lost original can defeat a valid will. The second is that marriage, divorce, and the birth of a child after the will was signed all have statutory consequences that vary by state and can partially rewrite the document by operation of law.
Two situations are worth an appointment rather than a form. The first is any estate where the answer is not obvious on the face of it - a blended family, a child with a disability whose means-tested benefits a direct gift would disturb, a business or farm that cannot sensibly be split, property in more than one state, a beneficiary with creditor or addiction problems, or an intention to leave someone out. Disinheriting a spouse in particular usually cannot be done by a will alone, because most states give a surviving spouse a statutory claim regardless of what the will says. The second is after a death, when you are holding a will and do not know what to do with it: the immediate practical questions are whether the original can be found, whether the estate must be administered at all, and what passes outside the will entirely - and those three answers determine everything that follows. Reviewing beneficiary designations at the same time as the will is the single highest-value hour in this area, because that is where the money usually is and it is the part a will cannot fix.
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