Common law
The common lawso named because it was "common" to all the king's courts across Englandoriginated in the practices of the courts of the English kings in the centuries following the Norman Conquest in 1066. The British Empire spread the English legal system to its historical colonies, many of which retain the common law system today. These "common law systems" are legal systems that give great precedential weight to common law, and to the style of reasoning inherited from the English legal system.
By-law
The Merriam-Webster Dictionary indicates that the origin of the word by-law is from the English word "bilawe", probably from Old Norse "*bȳlǫg", from Old Norse "bȳr" town + "lag-, lǫg" law. The earliest use of the term, which originates from the Viking town law in the Danelaw, wherein "by" is the Old Norse word for a larger settlement as in Whitby and Derby (compare with the modern Danish-Norwegian word "by" meaning town, or the modern Swedish word "by", meaning village). However, it is also possible that this usage was forgotten and the word was "reinvented" in modern times through the use of the adverbial prefix "by-" giving the meaning of subsidiary law or side-law (as in "byway"). In either case, it is incorrect to claim that the origin of the word is simply the prepositional phrase "by law."