They are both columns of the Ionic order, characterized by the ram's head capitals. The two buildings are entered by large staircases leading to multiple entrances. Greek figures are depicted on the exterior of each and are sculpted in high relief with deep undercuttings to enhance the figures. Another similarity is the size of the buildings. Both were built on a massive scale stretching out on a horizontal line.
Physical contrasts are apparent as well.
Although the Library of Congress has typical Greek traits, it does not have an engraved frieze like the altar of Pergamon. Wings were built on both monuments, but in different ways. The scenes depicted in sculpture are different also even though they both use Greek figures. At the Library of Congress, Poseidon, god of the sea, is sculpted. The altar of Pergamon, however, depicts a battle between the Gods and the Giants.