Some construction projects call for a larger masonry block than a standard brick, but solid concrete blocks can be very expensive and very heavy. One common compromise are largely hollow masonry blocks known as cinder blocks. Cinder blocks are also sometimes referred to as concrete blocks, breeze blocks, or concrete masonry units (CMUs), though these terms have nuanced differences among them. Cinder blocks are generally lighter than solid concrete blocks, which makes them easier for brick masons to place in position. The hollow spaces in cinder blocks also provide some natural insulation or allow reinforced concrete to be poured inside the rows of masonry.

Cinder blocks differ from concrete blocks in other ways besides their hollow design. Concrete blocks are made from a slurry of Portland cement and small aggregate, such as small stones or gravel. Cinder blocks, on the other hand, are made from a combination of Portland cement and cinders, the dusty remnants of burned coal. The result is a lighter weight block formed into a rectangular masonry block. Although many people use the terms "cinder blocks" and "concrete blocks" interchangeably, a true cinder block will always be lighter than a concrete block, and the texture of cinder blocks may be rougher than finished concrete blocks.

When bricklayers work with cinder blocks, they generally use techniques similar to standard brick laying. The alternative rows of cinder blocks are carefully offset so that the second layer stabilizes the first. A line of mortar is put down between cinder blocks, so the actual dimensions of a standard cinder block may be adjusted slightly to accommodate the mortar. Corners may be finished out with half blocks, or interlaced to create a four-cornered structure.

Because cinder blocks do not have a significant amount of tensile strength, concrete is often poured vertically into the hollow chambers to provide more stability and strength. An iron rod called rebar is often placed vertically in the hollow chambers as well to reinforce the poured concrete. It is not unusual to see rows of cinder blocks with lengths of exposed rebar on construction sites.

The problem with using reinforced concrete with standard cinder blocks is block placement. Standard cinder blocks must be carefully threaded over the rebar before they can be put into place. This may not be a problem for shorter projects such as home foundations, but it would be difficult and time consuming to thread individual cinder blocks over 20 feet sections of rebar. There is a solution to this problem, however.

Some cinder blocks, called speed blocks in the construction industry, are open-ended, generally shaped like the letter H. These speed blocks can be maneuvered around existing rebar and tilted into place by a skilled bricklayer. Standard cinder blocks are ideal building materials for small construction projects around the home or garden, but speed blocks are better suited for larger construction projects where significant amounts of reinforced concrete would be required.