Car transmissions come in two types, ones with fixed ratio gears, and ones which are continuously variable. When it comes to chemical bonds, we tend to think of them as being very much of the first type. Bonds come in fixed ratios; single, aromatic, double, triple, etc. OK, they do vary, but the variations are assumed as small perturbations on the basic form. Take for example the molecule shown below. The bonds as shown are all clearly single (the wedge and hashed bond are merely stereochemical notations). No-one would really think of drawing this molecule in any other way, and this idea of the transferability of bonds between molecules (all double bonds react in specific ways which are different from single bonds, and they also have characteristic spectroscopic properties, etc) is what allows molecules to be classified.