The x-axis is the first group's sample size (n1). The y-axis is the degrees of freedom (df). Adjust the second group's sample size, and each groups standard deviation, using the sliders. Key: Blue: equal (pooled) variances; [math]df = n_1 + n_2 - 2[/math] Green: unequal variances; Satterthwaite formula Red: unequal variances; smaller of [math]n_1-1[/math] or [math]n_2-1[/math]