systems like, for instance, limit cycle, we refer to [@arrowsmith1990]. In this paper, we always assume that an oscillator is a dynamical system having a limit cycle. We use the word clock when referring to a special type of oscillator described by the Andronov model [@OlMe].

Given a point $p_{0}$ in the limit cycle $\gamma$, the necessary time to return to $p_{0}$, after one round on the limit cycle, is the period $T_{0}$. A phase $\varphi$ is a real coordinate that describes the position of the representative point of the system on the limit cycle [@Nakao; @Pit].

Let $B_{\gamma}$ be the basin of attraction of the limit cycle. Consider the points outside the limit-cycle $\gamma$ but in $B_{\gamma}$. We extend the definition of phase to $B_{\gamma}$ as follows. We assign the same phase $\varphi$ to all points $p$ in $B_{\gamma}$ that converge to the same $p_{0}$ on the limit cycle $\gamma$ as $t\rightarrow\infty$, being the phase of $p_{0}$ precisely $\varphi$ [@gu1975]. The set of points $p$ that share the same phase is an *isochron curve*. If the oscillator’s states are on the same isochron at a given point in time, they continue to be on the same isochron in time 