| The first rule about Silicon Valley is: Do not try to define Silicon Valley.
At least not in one of the ba.* newsgroups, or you will be labeled as a troll.
Originally: north from San Jose, throughout Santa Clara county (as in tres equis's writeup, but he seems to have forgotten the city of Santa Clara itself).
More recently, its "definition" was changed to extend all the way north to the Dumbarton Bridge (California highway 84, east of 101), in order to include the city of Menlo Park as well.
Even more recently, this definition has become inaccurate, due to the increasing number of companies moving north into cities like Redwood City (especially its trendier Redwood Shores neighborhood) and San Mateo, so people troll about it actually extending all the way to the San Mateo Bridge (highway 92).
But it's just a name, and A rose by any other name would smell as sweet... | 
