What is the "collaboration" requirement for joint inventorship?

For two or more persons to be named as joint inventors, they must have collaborated in some way.  Two inventors who were unaware of each other's work could not be considered joint inventors, even if their efforts overlapped and even if the two inventors were employed by the same company (Kimberly-Clark Corp. v. Procter & Gamble Dist. Co., 973 F.2d 911, 916 (Fed. Cir. 1992)).  Similarly, if inventor B simply builds on the published work of inventor A, the result is the sole invention of inventor B, not the joint invention of A and B.  However, joint inventors need not have worked on the invention at the same time, nor are each inventor's respective contributions required to be equivalent in type or amount.  35 U.S.C. Section 116(a).