Some people respond by pointing out that Title 26 of the United States Code, which contains the tax laws, is not positive law. This is actually true, but it doesn't mean that Title 26 isn't law. Title 26 is the law, and the Internal Revenue Code is positive law. This point is explained on the basic page linked above, but here's some more detail on the relationship between positive law and the United States Code (of which Title 26 is a part).

Statutes enacted by a legislature such as the United States Congress are positive law. Congress enacted all the income tax statutes and these statutes are therefore positive law. For example, Congress enacted the Internal Revenue Code of 1986 on October 22, 1986. The Internal Revenue Code of 1986 is positive law.

All statutes ever passed by Congress, going all the way back to the First Congress in 1789, have been printed in a series of books called the Statutes at Large. So if you want to see the positive, statutory law of the United States, you can read it in the Statutes at Large.

However, there is a problem: statutes passed by Congress frequently amend previous statutes. When Congress passes a statute amending a previous statute, it doesn't necessarily re-enact the entire, changed statute. Instead, it may pass a statute striking out part of the previous statute and/or adding new material. Each such amending statute passed by Congress is also positive law.

Amendments create the following problem: how do you know what the current, effective law is? If all you ever look at is the positive law (i.e., the statutes passed by Congress, in the Statutes at Large), you'd have to start with an original statute (such as the Internal Revenue Code of 1986), and then look at every subsequent amendment, keeping careful track of every change Congress had made in the intervening decades, to find out what the law is today. That would be a ton of work.

Because of this problem, there is a strong need for a simple, easy way to access the current law, i.e., the law that is the final result of an original statute plus all the amendments that Congress has passed over time. And that's what the United States Code is. The United States Codes is a compilation of the statutes currently in effect, arranged by subject matter, with careful track having been kept of all amendments made over the years.

Here's how it works: when Congress first passes a new statute, the statute is codified, that is, it is added to the United States Code. Then, whenever Congress passes an amendment that consists of adding words to the statute, deleting words from the statute, or both, the keepers of the United State Code carefully strike out the parts of the Code that have been deleted and add any new material that Congress has added. The result is that the United States Code reflects current law.

To make this point clear, let's consider a simplified, hypothetical example. Suppose the original tax statute as passed by Congress simply said the following:

Statute 1 (S1) is passed by Congress and is therefore positive law. Let's imagine it remains in effect for 10 years. At that point Congress decides to amend the law to lower the tax rate. Congress could do this by repealing S1 and passing an entirely new statute. But instead of doing that, Congress passes the following S2:

S2, like S1, is positive law, because it was passed by Congress. The positive law in this example consists of S1 and S2. But what is the actual, effective law after the passage of S2? How is anyone supposed to compute their taxes?

The answer is that if you consider S1 and S2 together, you can see that the effective law after the passage of S2 is:

That's what you would really need to know to pay your taxes. The "effective law" shown above wouldn't be positive law, because Congress never passed it in that form. The positive law would be S1 and S2, which Congress did pass. But the effect of S1 and S2, considered together, would be the effective law shown above, and the effective law is what you would really need to know in order to pay your taxes.

Where would you find these different versions of the law? You would find S1 and S2 in the Statutes at Large. That is the official record of the positive law. You would find the effective law in the United States Code. When S1 was originally passed, it would have been put in the United States Code exactly as written. Then, after the passage of S2, the keepers of the United States Code would revise it to strike out "25%" and replace it with "20%" as provided in S2. So the United States Code would contain language which would result from statutes passed by Congress, even though Congress would never have passed one, single statute containing the language in that exact form.

In the simplified example presented above it wasn't hard to figure out what the effective law was by looking only at the positive law. If you just looked at S1 and S2 and put them together in your mind, you would get to the effective law. So there wasn't a strong need for the effective law in this example—you could figure out what you needed to know by looking only at the positive law, S1 and S2.

But imagine what things would be like if S1 were 100 pages long, and every subsequent amendment enacted into positive law was also many pages long, and there were dozens of amendments since the original statute was passed. Now, if you needed to know what the actual, effective law was today (because that's what you're supposed to obey), it would be hopeless to try to figure it out for yourself by looking only at the positive law. To get to the effective law, you'd have to start with the original piece of positive law, and look at every subsequent positive law amendment and keep careful track of every such amendment made by Congress. It would be too much work. Instead of painstakingly working through every relevant piece of positive law, wouldn't you rather just look at the effective law?

Fortunately, you can, because someone else has done the work for you. In real life, the original 1986 tax statute was over 100 pages long (almost 900 pages, in fact), and there have been dozens of long amendments passed since the original statute was enacted in 1986, so trying to figure out the effective tax law from scratch would be hopeless. Luckily, however, a group called the Office of the Law Revision Counsel of the U.S. House of Representatives has done the work for us.

The Revisers in that office keep track of the effective law. When Congress passed the Internal Revenue Code of 1986, the Revisers codified it. Then, each time Congress passed an amendment, the Revisers carefully struck out whatever parts of the previous statute Congress struck out, and added whatever new material Congress added. The result is the effective law.

And that's what Title 26 is. Title 26 is the tax title of the United States Code. Title 26 is not positive law. The positive law is the statutes as passed by Congress. But Title 26 is the effective law. Title 26 is a handy, convenient guide to the positive law.

Analogizing to the hypothetical example above, the Internal Revenue Code of 1986 and its many subsequent amendments are the positive law, like S1 and S2 in the example, and Title 26 is the effective law, like the "effective law after passage of Statutes 1 and 2" in the example. You can find the positive law in the Statutes at Large and the effective law in Title 26 of the United State Code.

If you want to figure out what the effective law is by looking only at the positive law, go right ahead, you are free to do so. You can start with the Internal Revenue Code of 1986 as passed by Congress and work through every amendment made over the subsequent decades, carefully keeping track of each change until you arrive at the effective law today.

But no one does that. It would be too much work. Instead, everyone simply looks at the effective law, contained in Title 26 of the United States Code.

So is Title 26 law? Yes, it is. It is not positive law. The positive law is the Internal Revenue Code of 1986 and all the subsequent amendments passed by Congress. Title 26 is the effective law. It is a convenient guide to the positive law. It is what you would get if you started with the positive law and carefully worked through it to get to the effective law.