One meshed gear pair reverses rotation direction. An even number of stages ends up spinning the same way as the input; an odd number reverses it.
Each external mesh is like two interlocking wheels rolling against each other — impossible for both to turn the same way. Miss this and a "clockwise" spec comes out backwards.
Not for an internal (annular) mesh, where the pinion and ring turn the same way — which is the whole basis of an epicyclic gearbox, and why a planetary set can reverse or not depending only on which member is held. A belt or chain drive also preserves direction unless it is crossed.
Estimate with the rule, then check it against the calculator that models it properly.
Open Gear Train Calculator →One meshed gear pair reverses rotation direction. An even number of stages ends up spinning the same way as the input; an odd number reverses it. Each external mesh is like two interlocking wheels rolling against each other — impossible for both to turn the same way.