Marginal distribution

It collapses the table down to a single variable, throwing away the information about the other one, which is why it is read off the margins. Every entry uses the grand total as its denominator, unlike a conditional distribution, which divides by one row or column total instead. Of 200 surveyed students, 80 are seniors and 120 are not, so the marginal distribution of class is $80/200 = 0.40$ senior and $120/200 = 0.60$ not senior. The proportions in a marginal distribution add to 1 across all categories of that one variable.