The wood thickness is another stumbling block for the beginner. Hardwood lumber is sold based on its thickness, the length of board will often vary and width of each board will almost always vary. Wood thickness is expressed in quarters of an inch in rough sawn thickness. You may have seen prices that look like this "4/4 Red Oak $3.25". In that example price the 4/4 refers to thickness of the wood. If you're thinking, hey isn't 4/4 just 1. In math yes, it most certainly is but in woodworking it is just called 4/4. In a lumberyard and in woodworking it is pronounced "four quarters", saying "1 inch" or "four fourths" might work but it isn’t how it is generally referred.
If you’re wondering why doesn't my 4/4 lumber measure a full 1 inch thick. The thickness refers to its rough sawn thickness. By the time you plane and joint the face of your board you are not likely to get a board much thicker than 3/4 of an inch. The more your wood is processed before you get it the closer it will be to 3/4 rather than 1 inch thick.
Other than the very common size of 4/4, there are other common sizes of lumber available. Some other sizes are 5/4, 6/4, 8/4, 12/4 and even 16/4. Not all wood is available in all sizes.