  The CKM matrix elements $V_{td}$, $V_{ts}$ and $V_{tb}$, while notoriously
hard to measure directly in experiment, can be determined to great precision
within the Standard Model (SM) with a minimal set of observables due to
unitarity of $V_{\rm CKM}$. We discuss the potential modifications of these
elements in extensions with non-unitary quark mixing matrices.
