Stable letter
= Stable letter
{title2=$t$}
A stable letter is the new generator of an <HNN extension> which implements an <isomorphism> between two specified subgroups by conjugation. In the convention $tat^{-1}=\phi(a)$, it moves elements of the source subgroup $A$ to the target subgroup $B$; using $t^{-1}at=\phi(a)$ reverses the naming convention.