主线

main line (of communication) / main thread (of a plotline or concept) / central theme

variant of 線|线[xian4]

thread / string / wire / line / CL:條|条[tiao2],股[gu3],根[gen1] / (after a number) tier (unofficial ranking of a Chinese city)

owner / master / host / individual or party concerned / God / Lord / main / to indicate or signify / trump card (in card games)