It is shown that the extended quadratic residue code of length 20 over 07157971630034X&_mathId=si1.gif&_user=111111111&_pii=S107157971630034X&_rdoc=1&_issn=10715797&md5=7ccd81d8de796d576d80c810291e5399" title="Click to view the MathML source">GF(7) is a unique self-dual 07157971630034X&_mathId=si16.gif&_user=111111111&_pii=S107157971630034X&_rdoc=1&_issn=10715797&md5=63cb9119871b41ec21fa9595386493c1" title="Click to view the MathML source">[20,10,9] code C such that the lattice obtained from C by Construction A is isomorphic to the 20-dimensional unimodular lattice 07157971630034X&_mathId=si13.gif&_user=111111111&_pii=S107157971630034X&_rdoc=1&_issn=10715797&md5=7e0778ea77038cc850698c4f427c88dc">07157971630034X-si13.gif">, up to equivalence. This is done by converting the classification of such self-dual codes to that of skew-Hadamard matrices of order 20.