Back
The back of a bookcase can be made out of the wood used for the rest of the bookcase, a piece of board or, as with built-in bookcases, can be made with the use of the house wall it rests against. The back of the bookcase is the base to which all other parts must be attached equally to ensure balance. Backs of bookcases are typically long and rectangular and must be cut with even dimensions. Cheaper bookcases may use board to cover the back of a bookcase, but rely on sturdy legs and shelves to hold the bookcase strong.
Legs
The legs of a bookcase sit on either side, closing the shelves in. The legs must be as long as the back and often a few inches longer to act as the agents upon which the bookcase stands. The legs must be made out of a sturdy wood such as oak or veneered plywood. Softer woods like pine may work in the short term, but loading a pine bookcase with many heavy books could compromise the strength of your bookcase.
Shelves
The shelves, ceiling and floor of your bookcase are all cut to the same length and connect from one leg to the other. Shelves are screwed into the legs and should be made from the same hard wood as the legs are made from. It is important to install shelves with equal strength into both legs to prevent the shelf from titling. The builder should take into consideration the height of the items going on her shelf to make sure she leaves enough space between each shelf to fit the items comfortably.
Supports
Shelves require supports to hold the weight of heavy books. Support are L-shaped and are installed underneath the shelf at the back of the bookcase. By using the back of the bookcase for strength, supports act as an arm to hold up the weight of the shelf and the items on top of it. Bookcases with cheaper board backings cannot use supports unless the support is installed against the legs of the bookcase.