Book shelves are divided into single-sided basic shelves, single-sided extended shelves, double-sided basic shelves and double-sided extended shelves. Because the single-sided basic frame can only be used on one side, it is usually placed against the wall or window and used on one side. Bookshelves can also be combined with single-sided extension frames. Double-sided basic frame can be used alone or combined with double-sided additional frame to form multiple connections. Single-sided and double-sided connection frames are only used when connecting and subtracting single-sided and double-sided basic frames.