"Google penance" is a term Carl Pyrdum of Got Medieval? uses to denote when he has written a post inspired by a Google search that brought a reader fruitlessly to his website.
For example, I get a fair number of hits from people looking for "Anglo Saxon riddle kids." I actually have an article on Anglo-Saxon riddles for children posted, so that's a good hit. But I also get people searching for "mi contra fa."
Now, if they're looking for this blog, that's a good hit. But if they're looking for information on the tritone, they are out of luck. I have a whole site named "Mi Contra Fa" and I never once explain what it means.
For this oversight, I do Google penance.
It's pretty simple in this case, since there's a Wikipedia article that summarizes most of the relevant information and debunks the "devil in music" thing at the same time. "Mi contra fa," or "the note mi played against the note fa" is the tritone, an augmented fourth or diminished fifth, the interval between (for example) F and B. It's dissonant - really dissonant - and it was considered good musical practice in the Middle Ages to avoid it. However, we don't have a single scrap of evidence to support the idea that people really believed that playing it summoned the devil or was an act of evil. It just sounded nasty.
Now, someone out there is going through their "Do, a Deer" and wondering how in the world mi and fa can be an augmented fourth apart. Do, re, mi, fa... that's a major or minor second, right? They are right next to each other! In what world does B follow F in a musical scale?
In the world of hexachords, and even then, not directly. Medieval musical theory wasn't based on octaves and major and minor scales. It was based on hexachords - six notes long, rather than the octave's eight - and modes. But yet, the octave was recognized as the perfect interval. How did you compose music if your 'octave,' this hexachord thing, didn't contain the octave?
You switched hexachords. There are three of them, see, each one patterned "tone-tone-semitone-tone-tone." That got you, in a world of mostly diatonic instruments:
Now, when you sing "Do, a Deer," you don't necessarily know what key you're in, right? You do know that the tonic, the base note that structures the piece, is going to be "Do." That's its name, no matter what note you're actually singing.
The same is true for each of the three hexachords. If I'm in the natural hexachord (the one starting on C), C is do. if I'm in the soft hexachord (the one starting on F), F is do.
Note that in the natural hexachord, F is fa. And in the hard hexachord (the one starting on G), B is mi.
Mi contra fa - augh, the tritone! There it is! And that's why in the soft hexachord, the only one that would naturally contain an F and a B, the B is flattened (or softened, and hence the name of the hexachord).