UnbakedDecimeter

@Xanne I agree that part of the confusion is what is essential. This is why I broke my question into parts. The answer to the 2nd question is contingent upon the 1st. Thinking about the 1st question helps answer the 2nd. I have a feeling that this is one of those areas in English which doesn't have a satisfactory answer. ☹

@BillJ Doesn't the appositive precede the subject in the first sentence on this website? literaryterms.net/appositive

@BillJ Why do you say that the appositive always follows the noun? There are many instances online that state differently.

@BillJ What's an NP?

@YosefBaskin No, there are many upper-left-hand keys. You are changing the meaning of the sentence. I don't understand how it's your view when you can't see my keyboard. 😆

@YosefBaskin In your sentence, you use a 2nd comma, but the appositive is restrictive. The upper-left key can be one of many.

I have a feeling that an appositive cannot precede a noun unless it's the very first thing that starts a sentence. And, if the appositive follows the noun and it's restrictive, I am guessing that you can use a comma if the situation requires once but you don't use the second comma.

So, in the last sentence, say I intended "the upper left key" to be the noun. Then, the appositive, "the ` key", is restrictive so it shouldn't have commas. However, the lack of a comma seems strange. Second, say that I intended "the ` key" to be the noun, do you put commas around "the upper left key"?