California Code, Vehicle Code - VEH § 5155

(a) Except as provided in Section 5161, the license plate for a passenger vehicle, commercial vehicle, or trailer shall provide a space not larger than two inches by three inches to the left of the numerical series and a space not larger than five-eighths of an inch in height below the numerical series for a distinctive design, decal, or descriptive message as authorized by this article. The license plates shall be issued in sequential numerical order or, pursuant to Section 5103, in a combination of numbers or letters.

(b) Specialized license plates authorized under this article may be issued for use on a motorcycle. That license plate shall contain a five-digit configuration issued in sequential numerical order or, pursuant to Section 5103, in a combination of numbers or letters. There shall be a space to the left of the numerical series for a distinctive design or decal and the characters shall contrast sharply with the uniform background color. A motorcycle plate containing a full plate graphic design is not authorized.

(c) Specialized license plates may be issued as environmental license plates, as defined in Section 5103.