The classical way of extending an $[n, k, d]$ linear code $\C$ is to add an overall parity-check coordinate to each codeword of the linear code $\C$. The extended code, denoted by $\overline{\C}$ and called the standardly extended code of $\C$, is a linear code with parameters $[n+1, k, \bar{d}]$, where $\bar{d}=d$ or $\bar{d}=d+1$. This extending technique is one of the classical ways to construct a new linear code with a known linear code and a way to study the original code $\C$ via its extended code $\overline{\C}$. The standardly extended codes of some families of binary linear codes have been studied to some extent. However, not much is known about the standardly extended codes of nonbinary codes. For example, the minimum distances of the standardly extended codes of the nonbinary Hamming codes remain open for over 70 years. The first objective of this paper is to introduce the nonstandardly extended codes of a linear code and develop some general theory for extended linear codes. The second objective is to study the extended codes of several families of linear codes, including cyclic codes, projective two-weight codes and nonbinary Hamming codes. Many families of distance-optimal linear codes are obtained with the extending technique.

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