Let $S$ be a set consisting of 10 elements. The number of tuples of the form $(A, B)$ such that $A$ and $B$ are subsets of $S$, and $A \subseteq B$ is __________.

To form an ordered pair of subsets $(A, B)$ of $S$ such that $A \subseteq B$, we consider the placement of each element $x \in S$.

For each element $x \in S$, there are 3 mutually exclusive and exhaustive choices:

1. $x \in A$ and $x \in B$

2. $x \notin A$ and $x \in B$

3. $x \notin A$ and $x \notin B$

(The case where $x \in A$ and $x \notin B$ is not allowed because $A \subseteq B$.)

Since $|S| = 10$, each of the 10 elements has 3 independent choices.