grammatical number - Meaning of the plural form of yarn

My dictionary defines the following:

yarn: thread that has been spun, used for knitting, weaving, etc.

I am unsure of the meaning of the plural form "yarns". To illustrate, I'll use two examples:

A. "He sold yarns of many colours---red, blue, and white."

B. "She found that five yarns corresponded to a width of one centimetre in the knitted fabric."

After consulting with people that speak English well, I get two different answers:

1. "yarns" refers strictly to several different types of yarn, as the word is used in example A. However, "yarns" can not be used to refer to several threads of yarn as in example B. (This makes the "yarns" somewhat similar to the "peoples".)




2. "yarns" may refer to both several different types of yarn and several threads of yarn. Both example A and B show correct usage of "yarns".

Which of these definitions of the plural form is correct? (Or maybe the answer is something different?) If "yarns" cannot be used to describe several threads of yarn (as in example B), then what should example B look like? Is the term "threads of yarn" ok?

The text I am writing is rather formal, so I would appreciate answers that are appropriate in a formal context.

Answer

I believe explanation 1 is the better guide.
When you're talking about the same yarn, you'd say "lengths of yarn", but if there were two different kinds of yarn in the same weave, you'd say "the two yarns are interwoven..."

Other examples of nouns that are pluralized only when different types or classes are being referenced: "I favour milder tobaccos", "they deal in plastics", "a large selection of fabrics".