Hi I thought I'd show a case where one might want to use diodes. In my logic, 'A' would indicated the normally opened contact an 'a' would be the normally closed contact for relay A. I'll use + to indicate OR and . to indicate AND. Lets say we wanted to create the equation: X=(A.B)+(C.D) We can stack the normally open contact of A in series with a B contact. We can then parallel these with the series of C and D. The output of these can be tied to the coil of X. Now say we also needed: Y=(A.B)+(E.F) We would have to used a new set of contacts for the A.B term because the wires work both ways. If we were using DC relays, we can add a pair of diodes in series with the two AND'd terms used to drive each X and Y. This reuses the same contacts to drive both X and Y. So, as you can see, the diodes are not necessary but can be used to save on the number of isolated contacts needed. One can also play boolian games with these equations to do things like reduce total power required. Since each relay is assumed to have both inverting and non-inverting outputs, one can change where things are inverted: y=(a+b).(e+f) If the case is that Y would be more likely to be active, this second equation makes more sense for power. You can see here that we have optimized the power without doing anything other than change a few wires. We may have also minimized the need for extra contacts and or diodes. Like I said, designing with relays takes a different way of looking at the problem. One can also see that the OR function can be created by putting contacts in series or in parallel, depending on the active state of the relay that one is driving. The same goes for the AND function. This adds another flavor to things because one can move which side of the relay one places the term to optimize, power, delay or number of contacts used. Dwight