Remember the common monomers:
\[
\boxed{
\begin{aligned}
&\text{Terylene}
&&\rightarrow
\text{Ethylene glycol + Terephthalic acid},
&\text{Glyptal}
&&\rightarrow
\text{Ethylene glycol + Phthalic acid}.
\end{aligned}
}
\]
Thus,
\[
\boxed{\text{Ethylene glycol}}
\]
is common to both polymers.