I–V–vi–IV is a path through a major key. I is home. V leans away. vi is the sad cousin. IV opens the door back toward home.
On this site the named loop is The Four Chords. The degrees are I, V, vi, and IV. In C major that sounds as C, G, Am, and F. In G major it sounds as G, D, Em, and C.
The loop is common because it can start bright and still dip. It is not magic, and it is not the only way a song can move.
A Roman number is a place in the key, not a fixed letter. I is the tonic chord. V is built on the fifth step. vi is the minor chord on the sixth step. IV is built on the fourth.
Change the key and the letters move. The feeling of the path stays. That is why a pattern page can offer a key chip while the degrees stay still.
Minor numerals are minor chords. vi is minor. I, V, and IV are major in a major key. If you play Am where the page says vi, you are in the right family.
Start the same four chords on a different beat and the mood shifts. Four Chords, from vi is vi–IV–I–V. It begins in the shade and climbs.
Three-chord Song drops the sad cousin and keeps I, IV, and V. Many early folk pages live there. Rock Three Chords is I–V–IV, a shorter walk.
A jazz turn such as ii–V–I Cadence is a different job. It aims at a landing. I–V–vi–IV is a wheel. You can ride it for a long time.
Open The Four Chords and tap through one key, then another. Then open a song you already hum, such as Amazing Grace or Ode to Joy, and see which cells match the wheel.
Not every public-domain page uses this exact loop. Silent Night stays close to a small major set. That is useful. You learn the wheel, then you notice when a song leaves it.
Play the loop slowly in A minor if you want the sad cousin to feel like home. The letters change again. The relative major is still close by.
The chord progression player opens the Songbook so you can jump from the pattern to a full page. Save a sketch in My Chords if you want the loop in your own name.
Do not chase a radio hit on this page. The pattern lists short references as theory hints. The playable thing is the loop itself.