The simple letters are:

Heh (ה).
Vau (ו).
Zain (ז).
Cheth (ח).
Teth (ט).
Yod (י).
Lamed (ל).
Nun (ן, נ).
Samekh (ס).
Ayin (ע).
Tzaddi (ץ, צ).
Qoph (ק).
See more about the simple letters or the Alef-Bet.