Sentence:  The dual <WSD>isogeny</WSD> of an isomorphism is its inverse. The dual of a composition of <WSD>isogenies</WSD> is the composition of the duals in the reverse order. If \varphi is an <WSD>isogeny</WSD> and \hat{\varphi} is its dual then the formulas \varphi(z+\Lambda) = mz + \Lambda', \varphi(z' + \Lambda') = (\operatorname{\varphi}/m)z' + \Lambda show that also / :::: \varphi\circ\hat{\varphi}=\left [\operatorname{deg}(\varphi)\right ]=\left [\operatorname{deg}(\hat{\varphi})\right ], / so that \varphi is in turn the dual <WSD>isogeny</WSD> of its dual \hat{\varphi}. <WSD>Isogeny</WSD> of complex tori, rather than isomorphism, will turn out to be the appropriate equivalence relation in the context of modular forms. 
Meaning of the word marked with <WSD> tags:  An epimorphism of group schemes that is surjective and has a finite kernel.