Cal. Code Regs. tit. 18 § 13601.2
Any valid written agreement extrinsic to a will which limits any absolute estate passing under the will may be considered together with the will in determining the true nature and scope of the estate. For example, if a wife upon her death devises her separate real property in fee to her husband pursuant to a previously executed written agreement that he, in turn, at his death will devise to her nephews such portion of the property as he did not consume during his life, in view of the agreement the husband will be deemed to take a life estate with power to use and consume the principal, and the nephews a remainder over in the unconsumed portion of the property.
NOTE: Reference: Section 13601, Revenue and Taxation Code.
Cal. Code Regs. Tit. 18, § 13601.2