large koto
瑟 refers to a large, ancient stringed instrument similar to a koto, known for its rich, resonant sound.
瑟 combines 玨 (two pieces of jade, suggesting precious stone chimes) with 必, which may have contributed a phonetic element. The character originally depicted a large zither-like instrument with many strings.
The top 玨 looks like two jade stone chimes, and the bottom 必 resembles the body and strings of a large instrument. Picture a grand koto made of jade, producing deep, resonant tones.
For シツ, imagine sitting down to play the large koto: sit -> シツ, as you sit and pluck the strings.