history
From 王勃's early-Tang poem 《送杜少府之任蜀州》 (Seeing Off Prefect Du on His Way to Shu). The couplet reads: 海内存知己，天涯若比邻 - 'Within the four seas, a true friend exists; at the ends of the earth, we are as next-door neighbors.' Written as a farewell to a friend taking a post in faraway Sichuan, remarkably upbeat for a Tang farewell poem.
usage
One of the most-quoted farewell / friendship lines in Chinese. Standard in emigration speeches, diplomatic toasts, alumni events, and telecoms / transport ad copy about 'shrinking the world'.
