A concrete masonry unit (CMU) is a standard-size rectangular block used in building construction. CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them. Those that use cinders (fly ash or bottom ash) as an aggregate material are called cinder blocks in the United States, breeze blocks (breeze is a synonym of ash) in the United Kingdom, and hollow blocks in the Philippines. In New Zealand and Canada they are known as concrete blocks (a name common in the United States also). In New Zealand, they are also called construction blocks. In Australia, they are known as Besser blocks or Besser bricks, because the Besser Company was a major supplier of machines that made concrete blocks. Clinker blocks use clinker as aggregate. In non-technical usage, the terms cinder block and breeze block are often generalized to cover all of these varieties. Composition Concrete blocks are made from cast concrete (e.g. Portland cement and aggregate, usually sand and fine gravel, for high-density blocks). Lower density blocks may use industrial wastes, such as fly ash or bottom ash, as an aggregate. Recycled materials, such as post-consumer glass, slag cement, or recycled aggregate, are often used in the composition of the blocks. Use of recycled materials within blocks can create different appearances in the block, such as a terrazzo finish, and may help the finished structure earn LEED certification. Lightweight blocks can also be produced using autoclaved aerated concrete; these are widely used for construction in Finland and similar northern countries for the material's inherent thermal isolation characteristics, as are expanded clay aggregate blocks. Sizes and structure The use of blockwork allows structures to be built in the traditional masonry style with layers (or courses) of staggered blocks. Concrete blocks may be produced with hollow centers (cores) to reduce weight, improve insulation and provide an interconnected void into which concrete can be poured to solidify the entire wall after it is built. Blocks come in modular sizes, with the most popular typically referred to (by their thickness) as "4-inch", "6-inch", "8-inch", and "12-inch". In the US, CMU blocks are nominally long and wide. Their actual dimensions are less than the nominal dimensions (to allow for -inch mortar joints between blocks in any orientation). In Ireland and the UK, blocks are usually excluding mortar joints. In Australia, New Zealand and Canada,