Reinforcing concrete masonry with steel rebar improves its structural capability by increasing its resistance to lateral loads. Though reinforced collar joints or cavities in multiple-wythe walls are possible, reinforcement more commonly is installed in block cells in single-wythe walls. Block designed especially for reinforcing make this work easier and more efficient. WESTERN STATES USE OPEN-END BLOCK In the western United States, particularly in seismic areas, reinforced masonry is the rule. Virtually all block sold in California are two-celled hollow block. Two-celled block increase the space available for rebar and grout. The cells align vertically when block are laid in running bond, thus creating continuous openings for rebar and grout. From 50% to 70% of these two-celled block are H-shaped or A-shaped block. H-block have a single web connecting the face shells. Both ends are open. A-block are similar but have one closed end.