CA Prob Code Section 100

(a)

Upon the death of a married person, one-half of the community property belongs to the surviving spouse and the other half belongs to the decedent.

(b)

Notwithstanding subdivision (a), a husband and wife may agree in writing to divide their community property on the basis of a non pro rata division of the aggregate value of the community property or on the basis of a division of each individual item or asset of community property, or partly on each basis. Nothing in this subdivision shall be construed to require this written agreement in order to permit or recognize a non pro rata division of community property.