Cinder block has a top and a bottom. Seems odd since it is a chunk of precast concrete, right? Well, there is a good reason for this. In the picture the block on the left is sitting correctly and the block on the right is upside down. The top of a block is wider, which allows mortar to be easily placed on it. The block on the right is noticeably narrower, which allows the edges to “nest” down into the mortar placed on the top of the previous row.

How can you tell top from bottom without looking each one over closely or using a tape measure? Notice the hashes (dark lines in the photo) in the center of the block on the right? They mark the bottom. So when you look at a row, or coarse, of block that has been installed you should not see any hash marks.