In the Eighties Adrian Mathias introduced the notion of soundness of an ordinal. An ordinal is sound if for any countable partition \(P\) of it only countably many ordinals are order-types of unions of subpartitionts of \(P\). Mathias showed that the least unsound ordinal \(\zeta\) is \(\omega_1^{\omega + 2}\) if \(\aleph_1\) can be embedded into the continuum but if \(\aleph_1\) is regular yet cannot be embedded into the continuum, \(\zeta \geqslant \omega_1^{\omega 2 + 1}\).
I am going to discuss his findings and consider the notion for the more general class of linear orderings building on work by him, MacPherson, and Schmerl. I am also going to mention some open problems. This is joint ongoing work with Garrett Ervin and Jonathan Schilhan.