I had thought that the rule was that odd houses are on the north or east sides of the street. Sometimes "north" is not where you think it is in Somerville, and some streets are just exceptions.
Turns out the rule is actually, odd numbers are on the right hand side of the street as you walk in the direction of increasing numbers. Most streets in Somerville are numbered with the low end at the south or east end of the street. But some streets are numbered the other way. Some of these are one way streets, but not all (Kidder St, for instance, is two way). Mostly when they're one way, the one way runs in the direction of increasing numbers, but not always. Some neighborhoods seem to have more of this kind of thing than others (south of Union Square and between Willow and College Ave in particular).
Who knew? At least now I can tell which house I'm looking at in Google Maps.