What does the following diagram represent?

Suppose, the following is a diagram of a protein's polypeptide chain:


[![enter image description here](https://i.stack.imgur.com/HWIPy.png)](https://i.stack.imgur.com/HWIPy.png)


What does this diagram represent?


1. What are the letters A, E, M, W, L, N, S, etc. represent? (I suppose these are amino acid codes. Right?)
2. Which one is the c-alpha atom here?
3. What are the sequence numbers `i-2`, `i-1`, `i`, `i+1`, `i+2`, etc. represent? Why there are negative and positive increments from `i`?


The following is a [related](https://chemistry.stackexchange.com/questions/153935/what-is-the-meaning-of-this-data-from-a-protein-data-bank-pdb-file) diagram:


[![enter image description here](https://i.stack.imgur.com/QthLx.png)](https://i.stack.imgur.com/QthLx.png)




1. Yes, those are 1-letter [amino acid codes](https://molbiol-tools.ca/Amino_acid_abbreviations.htm)
2. The image just shows each amino acid as a single point, so it doesn't the individual atoms, including the C−α.
3. This may require knowing where this figure came from to be more specific, but they seem to be interested in the distance between amino acids before and after some particular point in the chain.


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