LINE n.
Course of conduct, thought, occupation, or policy; method of argument; department of industry, trade, or intellectual activity. He is uncommonly powerful in his own line, but it is not the line of a first-rate man. Coleridge.
OPEN DOOR n.
ommercial intercourse open to all upon equal terms, esp. with reference to a nation whose policy is wholly or partially fixed by nations foreign to itself, or to territory newly acquired by a conquering nation. In this sense, often used adjectively, as, open-door system, open-door policy, etc.