Since all organic structures contain a hydrocarbon-like framework, it is often convenient to omit the carbon and hydrogen atoms altogether. Such representations are called line-angle structures or skeletal structures. Only lines representing C-C bonds are shown, while it is understood that a carbon atom lies at each vertex and endpoint. Carbon-hydrogen bonds are omitted. The number of implicit hydrogen atoms attached to each carbon is determined by subtracting the number of bonds on the carbon atom from four.