pattern - State the rule!

State the rule behind the following partition of letters, and fill in the questionmarks. (The puzzle has a clear and justifiable trivia-type solution.)

      A . . D E . G H I . K . . N O . . . S T . . . ? ? ?
   ---------------------------------------------------------
      . B C . . F . . . J . L M . . P Q R . . U V W ? ? ?

Answer

The rule appears to be:

The last letters of the names of U.S. states.

A

Alabama
Alaska
Arizona
California
Florida
Georgia
Indiana
Iowa
Louisiana
Minnesota

Montana
Nebraska
Nevada
North Carolina
North Dakota
Oklahoma
Pennsylvania
South Carolina
South Dakota
Virginia

West Virginia

D
Maryland
Rhode Island

E
Delaware
Maine
New Hampshire
Tennessee

G
Wyoming

H
Utah

I
Hawaii

Mississippi
Missouri

K
New York

N
Michigan
Oregon
Washington
Wisconsin

O
Colorao
Idaho
New Mexico

Ohio

S
Arkansas
Illinois
Kansas
Massachusetts
Texas

T
Connecticut
Vermont

So of the last three letters,

Y (Kentucky, New Jersey) is on the top row, X and Z on the bottom.