Cartographic maestro Andy Woodruff introduces us to the 13 original and official neighborhoods of Cambridge, and the reasoning behind them. He notes as way of an introduction: "If there’s one thing everybody knows about neighborhood boundaries in Boston, it’s that nobody knows where they are. But they’ll tell you you’re wrong if you try to draw lines." (Amen, brother.) The original 13 have evolved into different names since their inception more than 60 years ago (except maybe Neighborhood 9 and Area 4), but the boundaries remain fairly constant.
They were set based on civic institutions like schools. Take the above example from Woodruff re: Neighborhood 8, which is now Agassiz. An ideal neighborhood model is at the to and the neighborhood as it was designated, circling the school, below.