Left-side speech radical, the compressed form of 言. Marks 诚 as a speech-related virtue: sincere words, honest declaration. Same radical anchors 说 to speak, 话 talk, 谈 discuss, 谢 thank - and many moral-speech words like 谨 prudent, 谦 modest, fitting 诚 right in.
Right 成 supplies the sound - same chéng, same tone, a clean phonetic. It also lends meaning: 'words that come true / are made good' is the classical reading of 诚, sincerity as words backed by deeds. Sound family includes 城 city wall, 盛 abundant.
