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Obtaining the most from multiple-antenna relay systems requires algorithms that configure the source and relay adaptively to instantaneous channel conditions. In this paper, we define an antenna selection mode of operation as the number of selected transmit antennas at the source (which is equal to the number of data substreams), the substream-to-antenna mapping at the source, the number of selected transmit antennas at the relay, and the substream-to-antenna mapping at the relay. We develop dualmode and multimode antenna selection algorithms to choose the mode that is most likely to deliver the lowest vector symbol error rate assuming the overall data rate is fixed. The effective condition numbers of both the two-hop channel and the relay channel are derived to give intuition on how the spatial characteristics of the constituent channels affect mode selection and to derive low complexity algorithms. Link-level simulations show that our proposed algorithms usually select the best mode, thus improving the diversity performance of spatial multiplexing relay systems and providing array gains over the existing single-stream relay transmission strategies. The two-hop multimode algorithms are shown by system-level simulations to improve the reliability of transmission and extend spatial multiplexing capability to cell-edge users in a multi-cell network.