In general, state laws require all codicils to meet the same standards that a will must meet. These include being in writing, made by a testator--the person using the will to bequeeth property--who is of sound mind and who is at least 18 years old.
For example, Ohio Revised Code 2107.3 states that wills and codicils must be in writing and signed by the testator, or by someone else on behalf of and with the approval of the testator. The codicil must also be witnessed and ascribed to by two competent people in the presence of the testator.
An interesting aspect of codicils comes when they republish a previously revoked or invalid will. For example, Florida Statutes 732.5105 states that any effectively executed codicil that refers to a previous will effectively republishes that will with the modifications as described in the codicil.
For example, assume you previously drafted two wills. The first was drafted in 2001, and the second one, drafted in 2003, revoked the 2001 will. In 2010, you draft a valid codicil naming the 2001 will as your last will and testament. Executing this kind of codicil republishes the 2001 will, making it your last will and testament and not the later 2003 will. Even though the 2003 will is the most recent, the valid codicil supersedes it, reestablishing the 2001 will as the valid one.
A codicil, once made a part of a will, is just as valid and enforceable as the will itself. A codicil can only be revoked in the same manner as a will, and the codicil itself can serve to revoke another will or previous codicil.
For example, Connecticut Code section 45a-257 states that a person can revoke a will or codicil by destroying the document, directing someone else to destroy it, creating a new will or codicil or by having a probate court declare it invalid. If a will is revoked by the testator, any attached codicil is also revoked.
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