What is the period of $h(x)$?

Question

if $f_1(x)$ and $f_2(x)$ are periodic functions with periods $T_1$ and $T_2 $ respectively then we have $h(x)=f_1(x)+f_2(x)$ has period as,=

$$ \begin{cases} \frac{1}{2}LCM (T_1,T_2), & \text{if $f_1(x)$ and $f_2(x)$ are complementary pairwise comparable even functions.} \\ LCM (T_1,T_2), & \text{otherwise} \end{cases}$$

What are complementary pairwise comparable even functions?

I'm not getting how to start this even.Need some hint.