  For $\left(C(t)\right)_{t \geq 0}$ being a strongly continuous cosine family
on a Banach space, we show that the estimate $\limsup_{t\to 0^{+}}\|C(t) - I\|
<2$ implies that $C(t)$ converges to $I$ in the operator norm. This implication
has become known as the zero-two law. We further prove that the stronger
assumption of $\sup_{t\geq0}\|C(t)-I\|<2$ yields that $C(t)=I$ for all
$t\geq0$. Additionally, we derive alternative proofs for similar results for
$C_{0}$-semigroups.
