1. An implantable medical device, comprising: a sensor configured to provide signals indicative of a detected posture vector of a patient; a memory configured to store one or more defined posture vectors and to associate each defined posture vector with a tolerance describing a relationship with the defined posture vector; and a processor configured to determine, without any derivation of an angle, a respective similarity between the detected posture vector and each of one or more of the defined posture vectors and to classify a posture of the patient based on whether any similarity has a relationship to the respective defined posture vector that is described by the associated tolerance.