I’ll try to add a bit.
Historical Origins
The clearest way to distinguish between the two systems is their historical origins. The civil law system is by far the older legal system, dating ultimately back to the Roman Republic. As far as we can tell, it developed from a system of customary law and case-law in the Roman courts, but given the Roman passion for order and systemization, it was continually refined by various commentators into a comprehensive orderly system. The highwater mark of this effort during the late Roman Empire was the decision of Justinian I , Emperor of the eastern Empire, to order a comprehensive study and re-organization of the law. This resulted in the Corpus Juris Civilis, which is the foundation point for most subsequent civil legal systems. By the time the Corpus was produced, Roman civil law was already about a thousand years in the making - and English common law didn’t even exist yet.
However, because of the barbarian invasions, the civil law went into a hiatus period in western Europe. There were efforts to keep it going, but the local customs of the various barbarian invaders also became part of the law. It wasn’t until the beginnings of the Renaissance that the Italian universities began to study the Corpus again in detail, and it became widely known, in the second flowering of the civil law. It began to spread throughout western Europe, as part of the new learning.
The major exception to this pattern was England. The English common law has its roots in the customs of the various Anglo-Saxon groups that invaded and settled in England, but it really flowered under the Angevin kings. Henry II was a very strong king who centralised control in his royal government, including the royal courts. The royal courts were the place to go if you had not got justice in one of the local courts or the various feudal courts of the local lords. The royal courts, based in London, developed a single body of law, common to the entire kingdom of England, derived from the local customs. Some local customs continued to survive into modern times, but for the most part, the common law was the same throughout England. Feudal principles also had a major impact on the formative period of the common law, since the focus of the common law at that time was mainly the law of property, in particular land, which was held according to feudal principles.
Since the common law began to develop before the second flowering of the civil law, the English essentially re-invented the wheel, but with differences based on their local customs and court systems. Like the Roman origins of the civil law, the English common law developed from the customary law and court decisions - but the major methodological difference is that the common law did not get systematised in the same way that the Romans began to systematise their law. Common law remained rooted in court decisions, not legal commentaries. That methodological difference remains one of the significant points of difference between the two systems even today.
Code / Non-Code
It’s often said that the distinguishing feature between the two systems is the existence of a code. I would respectfully disagree. There’s no doubt that the civil law tends to favour broad, systematic statements of the law, with the Corpus Juris Civilis being the outstanding example. However, the Corpus is not itself a legal code in the sense of a statute. Part of it was a re-statement of imperial statues, but other parts were commentaries and instructional materials. As a whole, the Corpus was a comprehensive re-statement of the principles of the civil law, that has subsequently been adopted by various countries. That adoption could be by statute or by code, but it could also be by decisions of the local courts, accepting the Corpus as an accurate statement of the law.
France, for example, did not have a civil code until 1804, but its laws have always been based, at least in part, on the civil law. France was not a unified state until fairly late in the medieval period, and its constituent parts had their own legal systems, under the local feudal lords. The Duchies of Normandy, Brittany, Acquitaine, and Burgundy all had their own local laws, as did the royal territories such as the Île de Paris, which developed its own customary law, known as the coûtume de Paris. Each of these local areas had their own legal systems, based in part on their own customary law, with feudal accretions, as well as borrowings from the Corpus. Even after France had been unified, these local divisions and local legal systems continued. It wasn’t until the Revolution that there was a strong push to have a unified legal system for all of France, as part of the sweeping away of the old feudal divisions and powers of the upper classes. This movement culminated in the Code Napoléan of 1804. That was not the first civil code in Europe, but it was the most influential.
Similarly, Lower Canada (now Quebec) is a civil law jurisdiction, but it did not have a civil code until 1866. Prior to 1866, it relied on the coûtume de Paris as its statement of law, supplemented by the grand authors and court decisions. Lower Canada joined the ranks of codification in 1866, but its law had always been based on the civil law (except for a brief hiatus immediately after the Conquest).
Another example on a civil law jurisdiction without a code is Scotland. Scotland is considered a civil law jurisdiction, although Scots law has been heavily influenced by the English common law. But Scots law has its foundations in the civil law, in part because during the numerous periods when Scotland and England were at odds, bright young Scotsman went to France and to Holland for their education, including their legal education, where they learned civil law. However, Scotland has never codified its law, so its methodology relies on case law and commentaries, in a way similar to English common law.
Role of Statutes / Judiciary / Authors
There are also differences in the way common law and civil law rank the hierarchy of legal authorities. Both systems now give primary place to statue law, although that was a major point of contention in England. That came to a head during the Civil War and subsequent Restoration period, which firmly established statutes passed by Parliament as the primary law. The primacy of statute law was established in the civil law system much earlier, during the Imperial period.
The more interesting point is the difference in treatment between court decisions and legal authors. In the common law systems, court decisions are themselves statements of law, unless overturned by a higher court or modified by statues. In the civil law, a court decision is simply a comment on the law, deciding a particular case, but is not itself a statement of the law.
The two systems also take a different approach to commentaries by legal academics. In the civil law, the commentaries of legal authorities, such as Ulpian or Pothier, can be considered to be statements of law. By contrast, in the common law system, commentaries by academics do not have status as statements of the law. They are simply one person’s interpretation of the law.
Why the difference between the two? The civil law approach dates back at least as far as the Corpus, where the law was re-stated by eminent academics, and included commentaries by academics as authoritative statements of the law. It was also influenced by the fact that civil lawyers learned their law at universities, with the academic traditions of learned academics writing treatises. In England, common lawyers did not learn their law at the universities, during the formative period of the common law. They learned their law at the inns of court, by going to court and listening to the arguments and decisions. Treatises by academics in the common law system are thus not authoritative statements of the law.
Examples of Differences
Now, with all that said, what are the actual on the ground differences between the two systems? If you look at results in particular cases, my impression is that both systems will produce similar results in similar cases. That’s not a surprise - both systems have evolved in the western European traditions of liberalism and capitalism, so will reach similar results in cases dealing with individual rights and business matters. However, the way they reach those decisions may vary.
For example, both systems put a lot of emphasis on freedom of contract, but have technical differences in determining if a valid contract has been made. Both systems have the concept of “consideration” as essential to the formation of a contract, but have somewhat different definitions of what constitutes “consideration.”
Another example is the law of gifts. In the civil systems, gifts are a form of contract, and can be sued upon in court like other contracts. In the common law system, gifts are not considered a form of contract, and cannot be sued upon in the same way as in the civil system.
Overall, though, the metaphor I use is the difference between two different types of cars. A Ford and a Honda will both get you where you want to go, and both use internal combustion, but there are a lot of technical differences under the hood, and the parts they use are not inter-changeable.
Northern Piper, LL.B. (common law), LL.B. (civil law).