A pi bond is weaker than a sigma bond, but the combination of pi and sigma bond is stronger than either bond by itself. The extra strength of a multiple bond versus a single (sigma bond) is shown in many ways. The most obvious is a contraction in bond lengths. For example in organic chemistry, carbon-carbon bond lengths are in ethane (154 pm), in ethylene (134 pm) and in acetylene (120 pm). More bonds make the total bond shorter and stronger.