If you’re looking for an overview of 20th century American art, you could pick up a textbook and attend an art history class. Or you could save your tuition dollars and head to the National Gallery of Art to view its third exhibition drawn from the collection of the late Ruth Cole Kainen. Kainen, along with her husband, the artist Jacob Kainen, donated a whopping 2,000 works to the NGA over the years, including a final bequest of 781 works in 2012 that helped complete this exhibit. The show starts with representation (a 1920 etching of an East Hampton doorway by Childe Hassam) and wanders into all-out abstraction (an early Pollack, done on paper, and a 1976 Willem de Kooning, pictured). Refresh that stuff you learned in the American art class you took a decade ago, or at least refresh your cocktail-party art banter. The exhibition is on view Mondays through Saturdays 10 a.m. to 5 p.m. and Sundays 11 a.m. to 6 p.m. to Feb. 1 at the National Gallery of Art, 6th Street and Constitution Avenue NW. Free. (202) 737-4215. nga.gov.