If it is not clear where a property line is located, the owners on each side of the line may agree to establish a boundary. Joaquin v. Shiloh Orchards (1978) 84 Cal. App. 3d 192, 197. If they do so in a friendly and amicable manner, that should end the matter.
Where the doctrine often arises, in the context of disputes between neighbors, is when one neighbor believes that there is an agreed boundary and the other neighbor disagrees. For example, one neighbor may have assumed for the last twenty years that a fence between his house and the neighbors is built upon the boundary. The other neighbor, however, may have a survey done, discover that the fence is NOT on the real boundary, and the two may then have a dispute, which could result in litigation.
In such litigation, the agreed boundary doctrine could apply. This doctrine was most recently discussed by the California Supreme Court in Bryant v. Blevins (1994) 9 Cal. 4th 47, 36 Cal. Rptr. 2d 86. As the Court stated, “The doctrine requires that there be  an uncertainty as to the true boundary line,  an agreement between the coterminous owners fixing the line, and  acceptance and acquiescence in the line so fixed for a period equal to the statute of limitations or under such circumstances that substantial loss would be caused by a change of its position.” 9 Cal. 4th at 55, quoting Ernie v. Trinity Lutheran Church (1959) 51 Cal. 2d 702, 707.
The party who is arguing that the agreed boundary doctrine applies has the burden of proving all of its elements. In the Bryant decision, plaintiff argued that a barbed wire fence, which had stood for almost twenty years, proved that there was an agreed boundary. The Supreme Court disagreed. It found that the building of a fence, by itself, was not proof of any uncertainty about the location of the border. It reasoned that fences may be built for many reasons, not simply to mark borders. It held that, as rule, when there were accurate public records, at the County Recorder’s Office, which could be consulted to determine a border, the agreed boundary doctrine should not be used, absent very clear evidence of an agreement between the parties.