Linkage disequilibrium
The whole point about using D' rather than D is because D' is bounded between -1 an 1 for any allele frequency, while the bounds for D depends upon the allele frequency of interest.

Recall the definition of D

$$D = p_{ab} - p_ap_b$$

Let, $D_{min}$ and $D_{max}$ be the minimal and maximal values of $D$ for a specific allele frequency.

For $p_a = p_b = 0.5$, the  $D_min$ is when when $p_{ab} = 0$ ($D_{min} = -0.25$) and $D_{max}$ is when $p_{ab} = 1$ ($D_{max}=0.75$).

For $p_a = 0.8$ and  $p_b = 0.2$, the  $D_min$ is when when $p_{ab} = 0$ ($D_{min} = -0.16$) and $D_{max}$ is when $p_{ab} = 0.2$ ($D_{max}=0.04$).