"What Key Is This In?" — Four Ways to Answer

The short answer: the key is the note the song feels finished on. Fast routes to it: (1) check the last chord — songs overwhelmingly end on their home chord; (2) list the chords and match them to a key's chord family; (3) hum the note that feels like rest, then find it on the fretboard; (4) slide a scale shape until nothing clashes. Use two methods and let them confirm each other.

Method 1: The last chord (10 seconds, ~80% accurate)

Songs resolve. The final chord — or the chord the chorus keeps landing on with a "we're home" feeling — is the key most of the time. G at the end? Probably G major. Am? Probably A minor. This fails on fade-outs, deliberately unresolved songs, and anything tricky — which is why you confirm with...

Method 2: The chord family

Every key owns seven chords (why). For major keys, the fingerprint pattern is: I major, ii minor, iii minor, IV major, V major, vi minor (plus a rare diminished). So list the song's chords and find the key whose family contains them all:

G, C, D, Em → the family of G major (I, IV, V, vi).
Am, F, C, G → all in C major's family... but if Am feels like home, it's A minor (same family — see the tiebreaker below).

Two shortcuts within the pattern: the two majors a fourth apart (C and F, G and C, D and G) are usually IV and I or I and V; three majors in the song (G-C-D) are almost always I-IV-V with the key being the one they orbit.

Method 3: The hum test

Play the song, then stop it and hum the note your body wants to end on. Now hunt that note on the A or E string (this is where knowing the fretboard pays rent). That note is your tonic candidate; check whether major or minor of it matches the song's mood. This method feels unreliable until you try it — the resolution instinct is shockingly strong even in non-musicians.

Method 4: The slide test

Play a major scale shape softly over the song, starting anywhere. Clashes? Slide up one fret and try again. When every note agrees, read the root of your shape off the fretboard — you've found either the key or its relative. Crude, works, and doubles as ear training.

The major-vs-relative-minor tiebreaker

Methods 2 and 4 always end with two candidates — a major key and its relative minor share every note and every chord (C major / A minor, G major / E minor). Break the tie by gravity:

Which chord starts sections? Which one do phrases sink into?
Minor-home songs usually lean on the minor chord plus its own V (an E or E7 appearing in an Am song is a dead giveaway for A minor — that's harmonic minor's day job).
Mood is a hint, not proof — plenty of sad songs are in major.

Edge cases, honestly labeled

Key changes exist (that gear-shift final chorus), some songs are modal (one-chord grooves), and blues breaks the chord-family rules on purpose (all three chords are dominant 7ths — treat the I as the key and move on). If a song resists all four methods, it's usually one of these three situations, not you.