In particular, I've read that dihedral angles are calculated between two planes defined by the N, C-alpha and C' of each residue. I'm pretty certain that it's either the next carbon atom after C-alpha or the last carbon atom in the residue.

For protein residues I know that $C\alpha$ denotes the first carbon atom attached to a functional group. Each subsequent carbon is given a corresponding greek letter ($\beta, \gamma, \delta$) except for one. For example, each residue in a PDB file has a list of atoms that looks something like this:

• ATOM 32 N AARG
• ATOM 33 CA AARG
• ATOM 34 C AARG
• ATOM 35 O AARG
• ATOM 36 CB AARG
• ATOM 37 CG AARG
• ATOM 38 CD AARG

(A,B,G,D are stand-ins for the greek letters). Why is there just a plain C in this list? What is significant about that carbon?