Figure 2: Indexing conventions of hydrogen bonds.
From: Hydrogen bond rotations as a uniform structural tool for analyzing protein architecture
The flanking residues R1, R2, R3 and R4 around the hydrogen bond from Pi to Pj, the signed length Δ of hydrogen bonds along the backbone and the long-range L-group of hydrogen bonds corresponding to |Δ|>6. The rotations along the backbone 
indicate the rotations from the ith peptide unit Pi to the following Pi+1.
