The chord trinity known as I–IV–V is one of the most useful theoretical concepts for any musician. The I–IV–V is a skeleton key for countless songs in folk, country, rock, blues, and beyond, revealing the basic similarities of, say, “Louie Louie,” “Ring of Fire,” “Johnny B. Goode,” “Helpless,” “Three Little Birds,” “I Still Haven’t Found What I’m Looking For,” “This Land Is Your Land,” “Man of Constant Sorrow,” and “I Fought the Law.” Understanding I–IV–V progressions can help you jam along with songs you’ve never played before or change a song’s key without using a capo, and it can get you started writing your own songs, too. Learn what I–IV–V means and how these chords lay out in various keys, and play through some examples as used in classic songs, in this lesson for Acoustic Guitar (with tab and video). Here's a companion playlist of songs mentioned in the lesson.