Skanda Purana states:
Aasya madhya sthitha veda devadevasya bhasura? aksharam
tu dautam jihvayam tu Sivajnana
Vedas are the middle of the mouth, agamas are the tongue and the aksharas are the teeth of Lord Siva.
Every agama contains four parts: Kriya Pada (rituals, offerings, prana prasthita, etc.), Karya Pada (when/where bath should be taken, how to ...