What are Kerley (septal) lines?

The interstitium made visible by thickening of the connective tissue (interlobular septa)
Kerley A lines (A, apex): nonbranching deep septal lines 4 to 6 cm long, extending radially from the hilum into the upper lobes
Kerley B lines (B, base): horizontal lines less than 2 cm long, commonly found in the lower zone periphery; may be seen as high as the clavicles