The testator is the person who makes a will - the 'I' in every clause. When the will is admitted to probate, the court's first questions are about the testator: Was this really their signature? Did they have capacity? Were they free from pressure?
Capacity for a will is a modest standard: the testator must understand what they own, who their natural heirs are, and what the document does. But the standard must be met on the day of signing - which is why waiting on a diagnosis is how families end up litigating instead of grieving.
The vocabulary around it: the testator signs the will, witnesses attest it, the personal representative carries it out, and beneficiaries receive under it. Dying with a valid will is dying 'testate'; without one, 'intestate.'
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Educational information only, not legal advice. Terms have statutory details beyond these summaries - confirm your situation with a licensed professional.