644. What character can be added to both of these characters to give other characters? 言_ -> _ ⻌

Answer:

言  GEN, GON, i(u), koto - say, speech

⻌  The "Shinnyou" radical - road, walk, to advance

Add 周:言+周 周+⻌ 

周  SHUU, mawa(ri) - round, loop, lap

調  CHOU, shira(beru), totono(eru) - investigate, prepare

週  SHUU - week