Under Texas Inheritance Laws, a codicil is a supplement to a will. It can add to a will, revoke part of a will or explain parts of a will. If you want to make small changes to your will but you don't want to rewrite the whole will, you can make a codicil. A codicil republishes the original will. The will and codicil are then to be regarded as one instrument speaking from the date of the codicil. The codicil must refer to the original will to be effective. Because the codicil is so tied in with the original will, it must be executed with the same formalities required of a will. 280 / 731.
If the codicil was validly executed and republishes the prior will, proof of the codicil will operate also to establish the will without further proof. 320/660. I have previously written about some problems with codicils on my site https://inheritancelaws.info. Go to that site and search "codicil."
Update Texas Inheritance: In 2019, the dispute was over a codicil that referred to the original will "written in the summer of 2016." The original will was actually executed in February 2017. The court ruled that the codicil did not refer to the original will as required and the codicil was no good. 04-18-00355-CV