degrees of freedom with a mixed anova design

Hello - I am trying to figure out why my degrees of freedom are decimals rather than being whole numbers. I am running the mixed anova in R and the assumptions have all been met. The sample sizes of the two groups are unequal. Would this be an explanation? Or could it be that there is a correction applied? If you have any constructive comments or can point me to some resources that would be great. Thanks for your time.