Open chords are extremely useful and are standard knowledge for any guitar player.  There are five main open chord shapes, the CAGED shapes, so named for the chord each letter represents.  They are called open chords because they each utilize some of the open strings.  This is in contrast to barre chords, which prevent open strings from ringing and are covered in a different lesson.

ID 22 Open D Chords

Open Major Chords

If we take a look below we can see diagrams for each of these open major chords (Cmaj, Amaj, Gmaj, Emaj and Dmaj):

Watch the video below to learn how to identify the roots in CAGED major scales using the NANDI method: