Sears, Roebuck and Company (defendant) sold washing machines to two sets of plaintiffs. Each set of plaintiffs filed a products-liability suit against Sears. The first set of plaintiffs alleged that the washing machines caused mold. The second set of plaintiffs alleged that the washing machines suddenly stopped for no reason. Each set of plaintiffs filed a motion for class certification. Whirlpool, the manufacturer of the washing machines, had made certain design changes to the mold-causing machines. Sears argued that these modifications made different models defective in different ways, thus making class certification inappropriate. The district court denied the mold plaintiffs class certification and granted the stoppage plaintiffs class certification. The mold plaintiffs and Sears appealed.