A codicil is an addition or supplement to a will. Someone may have been born or died since the original will was executed. A codicil is an easy way to make additions or deletions to a will without having to redo the entire will. However, the codicil has to be executed with the same formalities as a will. I have also previously written that when properly executed a codicil republishes the will as of the date the codicil is executed.
Consider this situation: a man executes a will; several years later, he executes a second will that revokes the first will; then, several years later, he makes a codicil that references the first will and, like all codicils, says that the first will is republished. Is the first will revived even though the second will revoked it? Yes according to the Texas courts. A properly executed codicil has the effect of validating and republishing the prior will so that the will and codicil will then be considered as one instrument speaking from the date of the codicil. 460 S.W.2d 215.
Going even further, most jurisdictions in which the question has arisen also hold that a properly executed codicil validates a prior will which was inoperative or invalid because of defective execution, lack of testamentary capacity, or undue influence. 280 S.W.2d 731; No. 06-08-00015-CV, Texarkana.