Pen. Code, § 1016

There are six kinds of pleas to an indictment or an information, or to a complaint charging a misdemeanor or infraction:

1. Guilty.

2. Not guilty.

3. Nolo contendere, subject to the approval of the court. The court shall ascertain whether the defendant completely understands that a plea of nolo contendere shall be considered the same as a plea of guilty and that, upon a plea of nolo contendere, the court shall find the defendant guilty. The legal effect of such a plea, to a crime punishable as a felony, shall be the same as that of a plea of guilty for all purposes. In cases other than those punishable as felonies, the plea and any admissions required by the court during any inquiry it makes as to the voluntariness of, and factual basis for, the plea may not be used against the defendant as an admission in any civil suit based upon or growing out of the act upon which the criminal prosecution is based.

4. A former judgment of conviction or acquittal of the offense charged.

5. Once in jeopardy.

6. Not guilty by reason of insanity.

A defendant who does not plead guilty may enter one or more of the other pleas. A defendant who does not plead not guilty by reason of insanity shall be conclusively presumed to have been sane at the time of the commission of the offense charged; provided, that the court may for good cause shown allow a change of plea at any time before the commencement of the trial. A defendant who pleads not guilty by reason of insanity, without also pleading not guilty, thereby admits the commission of the offense charged.