666. Which of the points $A(5 / 8)$ and $B(8 / 13)$ is to the left? (The coordinates of the points are given in parentheses; the positive direction is to the right.)
$\triangle$ Coordinates increase from left to right, so we need to compare the fractions $5 / 8$ and $8 / 13$ in terms of size:

$$
\frac{5}{8}=\frac{5 \cdot 13}{8 \cdot 13}=\frac{65}{8 \cdot 13}>\frac{64}{8 \cdot 13}=\frac{8 \cdot 8}{8 \cdot 13}=\frac{8}{13}
$$

This means that the coordinate of point $A$ is greater, so point $B$ is to the left.