California Code, Elections Code - ELEC § 13116

(a) In an election at which state, county, city, or other local measures are submitted to a vote of the voters, all state measures shall be numbered in numerical order, as provided in this chapter or division. All county, city, or other local measures shall be designated by a letter, instead of a figure, printed on the left margin of the square containing the description of the measure, commencing with the letter “A” and continuing in alphabetical order, one letter for each of these measures appearing on the ballot.

(b) An elections official may commence designating local measures with any letter of the alphabet following the letter “A,” and continuing in alphabetical order, in order to avoid voter confusion that might result from different local measures carrying the same letter designation in successive elections.

(c) Where two or more counties or cities submitting measures to the voters are in close proximity, the elections officials of those counties or cities may mutually agree to use letter designation for ballot measures that will not conflict or confuse the voter.