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Ternary channels can be used to model the behavior of some memory devices, where information is stored in three different levels. In this paper, error correcting coding for a ternary channel where some of the error transitions are not allowed, is considered. The resulting channel is nonsymmetric, therefore, classical linear codes are not optimal for this channel. We define the maximum-likelihood (ML) decoding rule for ternary codes over this channel and show that it depends on the channel error probability. An alternative decoding rule which depends only on code properties, called dA-decoding, is then proposed. It is shown that dA-decoding and ML decoding are equivalent, i.e., dA-decoding is optimal, under certain conditions. Assuming dA-decoding, we characterize the error correcting capabilities of ternary codes over the nonsymmetric ternary channel. We also derive an upper bound and a constructive lower bound on the size of codes. The results arising from the constructive lower bound are then compared, for short sizes, to optimal codes (in terms of code size) found by a clique-based search. It is shown that the proposed construction method gives good codes, and that in some cases the codes are optimal.