He has studied the prosody of Tang poetry for years.
Tips
history
格律 governs 近体诗 (regulated verse) crystallized in the Tang dynasty: fixed line count (4 or 8), character count per line (5 or 7), tonal patterns of 平/仄, strict rhyme, and parallel couplets in 律诗. 古体诗 (ancient-style verse) is by contrast largely free.
usage
Compare 格律诗 (regulated verse) with 自由诗 (free verse) — modern poets often abandon 格律 entirely.