Techniques for posture classification of a patient in a coordinate system of a sensor. According to one aspect, a defined vector is obtained from a sensor disposed in a substantially fixed manner relative to the patient. The defined vector is described in a coordinate system of the sensor and without regard to an orientation in which the sensor is disposed in relation to the patient. A detected vector is obtained from the sensor that is described using the coordinate system of the sensor. The detected vector and the defined vector to are used to classify the posture state of the patient without regard to the orientation in which the sensor is disposed in relation to the patient. A response may be initiated by a medical device, which may include adjusting therapy delivery.
