In most states, a validly executed Codicil attached to the Will republishes the Will so that the Will and the Codicil take effect on the date of the codicil. This means the Will and all prior Codicils are treated as if they were re-executed as of the day the Codicil was executed. A Will republished by a Codicil is interpreted according to the law in effect at republication, not according to the law in effect when the Will was originally executed.
A validly executed Codicil validates an improperly executed or otherwise invalid Will if the Codicil refers to the earlier invalid Will with sufficient certainty to identify and incorporate it or if the Codicil is on the same paper as the invalid Will.
In most states, a codicil may be self-proved in the same manner as a Will.