Hydropathy is the free energy change that occurs when transferring an alpha helix from a hydrophobic to hydrophilic environment. A negative hydropathy indicates a hydrophilic alpha helix, and a positive hydropathy indicates a hydrophobic environment.
Sequences of around 20 residues can be studied, allowing the identification of transmembrane helices.
This process to identify transmembrane helices can only be used to identify alpha helices. Beta barrels, due to differences in their structure, cannot be identified in this way.