The word "law" is ultimately derived from the plural form of the Old Norse word lag which had the sense of laying in order.  The Old Norse word became the Old English word for law, lagu.  Thus, the phrase "laying down the law" essentially means to lay down what is has been laid down.  Those using the phrase need not worry about redundancy, however, because the California Civil Code conveniently provides that "superfluity does not vitiate".  Cal. Civ. Code § 3537.

By statute, California recognizes two types of laws - organic or ordinary.  Cal. Code Civ. Proc. § 1895.   Laws may be either written or unwritten.  Id.  The organic law is the Constitution, which is "altogether written".  Cal. Code Civ. Proc. § 1897.  Other written laws are denominated statutes and thus California's written laws consist of its constitution and statutes.  Id.  

Unwritten laws are not promulgated and recorded.    Cal. Code Civ. Proc. § 1899.  While they may have no certain home, they are nevertheless observed and administered in the courts.  Id.  Unwritten laws are "collected from the reports of the decisions of the Courts and treatises of learned men".   Id.  See also Cal. Evid. Code § 160 ("'Law' includes constitutional, statutory, and decisional law.") and Cal. Gov't Code § 811 ("'Law' includes not only enactments but also the decisional law applicable within this State as determined and declared from time to time by the courts of this State and of the United States.").  

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