In this brief note we remark that the four spatially resolved relativistic radio jets among the 14 dynamically confirmed black hole x-ray novae are all in systems with long orbital periods. Many shorter period systems show transient radio outbursts which are attributed to jets, but these jets have not been resolved. Super-Eddington accretion has been suggested as required for jet formation, but the apparent outburst luminosities of these 4~systems are not all super-Eddington (and some Eddington outbursts did not form jets). We speculate that some as yet unknown process favors formation of substantially larger jets in these systems. Two short period systems show evidence for extent, but have not been resolved into multiple components as the long period systems have.