The secrets of Hebrew calendars: what are the two months of the year that differ from the others?

It turns out there are three options for the length of these months:

The patterns vary from year to year, and in the upcoming year (5786) we’re going to see this:

Based on the Rebbe’s explanations on Shabbat Parshat Nitzavim 5751, Rosh HaShanah and Shabbat Parshat Vayeilech, 5752(1991).