PID: P+D or P-D

I have a light grasp on the PID control concept. There is one portion of it that I have seen presented with contradictory information. Currently, I understand the purpose and function for all three parameters: P, I & D. In the control system we will be using, we plan on just using a PD loop.

Here is the question: I have seen the equation represented as (PerrorKP)+(DerrorKD), I also have seen it as (PerrorKP)-(DerrorKD). So which is it, and when would I use one or the other?:confused:

D should take off your attack, so it should be P - D. If Kd is negative, P+D works too.