In his book Everyday Tonality, Philip Tagg proposes the concept of a "permanent Picardy third":
One of the most common alterations in non-classical tertial harmony is to raise the third of tonic triads in minor modes. ... Such alteration can be understood in terms of a tierce de Picardie used constantly throughout a piece of music as substitute for the tonic minor triad (p. 274-275).
The title track to The Beatles' Sgt. Pepper's Lonely Hearts Club Band is a good example.
Tagg would argue that this song is in G dorian with a permanent Picardy third. But I have never liked this interpretation. I don't see what's wrong with calling this G major with a bIII (B-flat major) thrown in for a little harmonic color and spice. Yes, B-flat isn't in the key of G major - and ya know what? That's okay!
The permanent Picardy strikes me as the musical equivalent of literature's "unreliable narrator", in which a story is told from a compromised perspective. In music, the tonic triad determines whether the composition in major or minor. To argue otherwise is to adopt an "unreliable tonic" principle that seems to me like a theorist who is trying too hard to have something meaningful to say! But then this morning I discovered the song 'Somewhere I Belong', the third track from Linkin Park's 2003 album Meteora.
In this case, the song is CLEARLY in B-flat minor (I don't see how anybody could argue with that), and yet the tonic triad in the chorus is consistently major. It's the first example I've ever encountered that thoroughly convinces me of Tagg's "permanent Picardy" concept. So while I'm still skeptical of the principle, and while I vehemently disagreement with Tagg's assertion that the permanent Picardy third is "one of the most common alterations", there are compelling examples out there - you just have to look pretty hard to find them!
Aaron Krerowicz, pop music scholar
An informal but highly analytic study of popular music.