Kerly lines result fomr thickened connective tissue planes occurring as a result of pulmonary edema or carcinomatosis.
  
Kerly A Lines: Radiate towards hila in mid- and upper-lung zones, 3-4 cm long, smaller than vascular markings (not useful)
Kerly B Lines: Horizontal, <2 cm long an d1 mm thick, at periphery of lung, reach lung edge (very useful)
Kerly C Lines: Thickened interlobular septa