Understanding the evolution of the San Andreas transform is a key not only to broad aspects of regional geology but also to the development of specific structural provinces. Though stable in the regional sense, the paired triple junctions at the ends of the transform have had transient unstable configurations whereever the trends of the prior trench and the newly developing transform were locally not collinear. The resulting instabilities were mainly of kinds inferred to induce extensional tectonics within a nearby region. Passage of the Mendocino fault-fault-trench triple junction northward along the central California coast coincided well with pulses of initial subsidence in Neogene sedimentary basins near the continental margin and with eruptions at local volcanic centers in the Coast Ranges. Passage of the Rivera ridge-trench-fault triple junction southward was associated with the rifting events that formed the California Continental Borderland and the Gulf of California.