\begin{definition}[Definition:Commensurable/Notation]
There appears to be no universally acknowledged symbol to denote commensurability.
{{AuthorRef|Thomas Little Heath|Thomas L. Heath}} in his edition of {{BookLink|Euclid: The Thirteen Books of The Elements: Volume 3|Sir Thomas L. Heath|ed = 2nd|edpage = Second Edition}} makes the following suggestions:
:$(1): \quad$ To denote that $A$ is commensurable or commensurable in length with $B$: 
:::$A \mathop{\frown} B$
:$(2): \quad$ To denote that $A$ is commensurable in square with $B$: 
:::$A \mathop{\frown\!\!-} B$
:$(3): \quad$ To denote that $A$ is incommensurable or incommensurable in length with $B$: 
:::$A \mathop{\smile} B$
:$(4): \quad$ To denote that $A$ is incommensurable in square with $B$: 
:::$A \mathop{\smile\!\!-} B$
This convention may be used on {{ProofWiki}} if accompanied by a note which includes a link to this page.
\end{definition}