The term "cross pieces" refers to the horizontal supports that run perpendicular to the main structure, typically used in the construction of furniture, bridges, and buildings. The antonyms for this term are the vertical supports or posts that run parallel to the main structure, as well as the diagonal supports that help to strengthen the structure in different directions. Other antonyms include the single beams or planks that run the length of the structure without any intersection with other pieces. These pieces provide stability and rigidity to the overall structure and are essential elements in the construction of many architectural designs.