Tablespoons of butter.

A tablespoon is a unit of volume. Namely, it is one-sixteenth of a cup.

Now, there are two distinct units called “ounces” that are commonly used in the United States. One is the avoirdupois ounce, also know as the United States customary ounce, which is a unit of weight; it is one-sixteenth of a pound. The other is the U.S. customary fluid ounce, which is a unit of volume; it is one-eighth of a cup.

One-sixteenth of a cup is a tablespoon. One-eighth of a cup is an ounce (fluid). One-eighth of one-half of a cup is a tablespoon. These are all measures of volume.

Butter, however, is sold by weight:

The 16-oz. package of butter in the photo above says that it contains four sticks. This is one stick:

The stick is divided into eight “tablespoons”.

But the “tablespoons” of butter depicted above are not one tablespoon each in volume. And there is no such thing as a unit of weight called the “tablespoon”.

So what is this? Well, one stick of butter is 4 ounces in weight. 8 ounces in volume is one cup. By analogy, if we think of 8 ounces in weight as a “cup” in weight (which is not actually a real weight unit!), then one-sixteenth of that weight is a “tablespoon” in weight (by analogy with one-sixteenth of a cup in volume being a tablespoon in volume). Neither cups nor tablespoons are real weight units! But if we call an 8-oz. weight a “cup”, then we can call a 1/2 oz. weight a “tablespoon”.

Sticks of butter are divided into metaphorical tablespoons of butter.