There is no internationally recognized standard for the Jurassic/Cretaceous boundary; a precise biostratigraphical correlation between different paleobiogeographic provinces is impossible due to barriers blocking faunal exchange. This problem may be overcome by using magnetic polarity events, which are globally isochronous. The magnetic polarity time scale of the latest Jurassic and earliest Cretaceous enables determination of the relative timing and age span of several possible definitions of this system boundary among different biostratigraphies within the Tethys-Atlantic faunal realm; therefore, the polarity scale can be used as a regional standard for this interval of time. The base of reversed polarity chron M18 falls in the middle of the interval covered by various recent definitions of the Tethyan Jurassic/Cretaceous boundary. If polarity chron M18 also proves to be recognizable in the Boreal realm, then its basal magnetic reversal could serve as a precise, globally correlatable event for defining the Jurassic/Cretaceous boundary.