A scalar quantity is a physical quantity given by a single number (and its physical dimension) that is the same for all observers. The type of physical quantity (scalar, vector, tensor) is determined by how the quantity behaves when passing from one frame of reference to another (when passing from one observer to another). Physical quantities are in relation to each other by physical laws. Physical laws then have the same form in all frames of reference, which is the content of general covariance. In the basic curriculum, this principle is less rigorous.