1/8th of a cup is 2 tablespoons.

But I prefer to measure butter (and most other things -- especially sticky/messy/powdery things) by weight because it's easier. 1 cup of butter weighs about 227 grams, so 1/8 cup is about 27 grams.

In the US, butter is sold in sticks with 1/8 cup markings on the wrapper

Ehy would you measure butter in cups, you dont drink it

Most American cookbooks use volume measures for baking because most of us don’t own scales. It sometimes leads to wildly inconsistent results when an ingredient is compacted too much.

Yeah even the cup measurement itself varies. It's sometimes 250mL and sometimes 236mL and sometimes 227mL.

Cups is a measure of volume. They could just as easily print cc markings, but I don't think they do.