Tanya for the 29th of Elul, Erev RH כ''ט אלול and Rosh Hashana ראש השנה

Only the lower four Supernal Sefirot can extend to the lower worlds. Letters include both unique life force from Hashem as well as the physical sound that they make, combining to create words and meaning. Words, similarly, are rooted in thought, and physically evolve into words that are made of sounds.