A common subsequence of a collection of strings is a subsequence of all the strings in the collection. For example, $\textrm{ACGT}$ is a common subsequence of the two strings $\textbf{AC}\textrm{AA}\textbf{GT}\textrm{AA}$ and $\textrm{C}\textbf{A}\textrm{GTGC}\textbf{CGT}\textrm{TA}$. Note that $\textrm{ACGT}$ is not a longest common subsequence of these two strings, as $\textrm{ACGTA}$ appears in both strings and is one symbol longer.