Æ code[1] 

Code defined in a single angular-momentum subspace that is embedded in a larger direct-sum space of different angular momenta, which can arise from combinations of spin, electronic, or rotational, or nuclear angular momenta of an atom or molecule. A code is obtained by solving an over-constrained system of equations, and many solutions can be mapped into existing codes defined on other state spaces.

A simple example of an Æ code is the error-detecting code with codewords \begin{align} \begin{split} |\overline{0}\rangle&=\frac{1}{\sqrt{2}}\left(|J,-m\rangle+|J,m\rangle\right)\\ |\overline{1}\rangle&=|J,0\rangle~, \end{split} \tag*{(1)}\end{align} constructed out of states of total angular momentum \(J\) and its projection \(m\) for any \(J,m\geq 2\). This code detects a single change in \(m\) or \(J\).