The physical meaning of is also well known; namely, any velocity which in any time-system is of magnitude is of the same magnitude in every other time-system. No assumption of the existence of a velocity with this property or of the electromagnetic invariance has entered into the deduction of the kinematic equations of the hyperbolic type. A velocity greater than cannot represent any time-system, and accordingly its physical significance must be entirely different from that of a velocity less than . 52.5 It is easily proved from (ii) of 52.2 that If the origin and the event-particle , i.e. (), be co-momental and be the time-system whose moment , contains , then by (i) If and be sequent and on a point-track, and be the time-system whose point is occupied by , then by (i) Thus there are three ways in which the 'separation' between two event-particles ( and ) can be estimated; namely, (1) in any assumed time-system the -distance between the -points occupied by the event-particles measures -space separation: (2) the lapse of -time between the -moments occupied by the event-particles measures -time separation: and (3) if the event-particles be co-momental, measures the 'proper' space separation and there is no 'proper' time separation; and if the particles be sequent, measures the 'proper' time separation and there is no 'proper' space separation. In the framing of physical laws it is essential to consider what measure of separation is relevant. It is to be noted that there may be time-systems (other than ) of special relevance to the phenomena in question. It is not at all obvious that invariance of form in respect to all time-systems is a requisite in the complete expression of such laws; namely, the demand for relativistic equations is only of limited applicability. If and be on a null-track Event-particles on the same null-track may be expected to have special physical relations to each other. Call such event-particles 'co-null.' 52.6 We may conceive a special time-system associated (by some means) with each event-particle (). Thus is a function of these four co-ordinates of a particle; or in other words, () are functions of (). A correlation of time-systems to event-particles which is one-many, so that there is one and only one time-system corresponding to each event-particle, is called a 'complete kinematic correlation.' The portion of that correlation which only concerns event-particles at the time is called a 'kinematic -correlation.' Other portions can be selected by confining the event-particles to certain regions in the -space.