Given two distinct numbers $ b_1$ and $ b_2$, their product can be formed in two ways: $ b_1 \times b_2$ and $ b_2 \times b_1.$ Given three distinct numbers, $ b_1, b_2, b_3,$ their product can be formed in twelve ways: 

$ b_1\times(b_2 \times b_3);$ $ (b_1 \times b_2) \times b_3;$ $ b_1 \times (b_3 \times b_2);$ $ (b_1 \times b_3) \times b_2;$  $ b_2 \times (b_1 \times b_3);$ $ (b_2 \times b_1) \times b_3;$ $ b_2 \times(b_3 \times b_1);$ $ (b_2 \times b_3)\times b_1;$ $ b_3 \times(b_1 \times b_2);$ $ (b_3 \times b_1)\times b_2;$ $ b_3 \times(b_2 \times b_1);$ $ (b_3 \times b_2) \times b_1.$ 

In how many ways can the product of $ n$ distinct letters be formed?
To determine the number of ways to form the product of \( n \) distinct numbers, we need to consider both the order of multiplication and the different ways to parenthesize the product. 

1. **Base Case:**
   - For \( n = 1 \), there is only one way to form the product, which is the number itself. Thus, \( a_1 = 1 \).

2. **Recursive Step:**
   - For \( n > 1 \), we need to consider the different ways to parenthesize the product and the different orders in which the numbers can be multiplied.
   - The number of ways to parenthesize \( n \) numbers is given by the \((n-1)\)-th Catalan number, \( C_{n-1} \). The Catalan number \( C_k \) is given by:
     \[
     C_k = \frac{1}{k+1} \binom{2k}{k}
     \]
   - For each parenthesization, there are \( n! \) ways to order the \( n \) numbers.

3. **Combining the Results:**
   - The total number of ways to form the product of \( n \) distinct numbers is the product of the number of ways to parenthesize the numbers and the number of ways to order the numbers.
   - Therefore, the number of ways to form the product of \( n \) distinct numbers is:
     \[
     a_n = C_{n-1} \times n!
     \]
   - Substituting the formula for the Catalan number, we get:
     \[
     a_n = \frac{1}{n} \binom{2(n-1)}{n-1} \times n!
     \]
   - Simplifying further, we have:
     \[
     a_n = \frac{(2(n-1))!}{(n-1)! (n-1)! n} \times n!
     \]
     \[
     a_n = \frac{(2n-2)!}{(n-1)! (n-1)!} \times \frac{n!}{n}
     \]
     \[
     a_n = \frac{(2n-2)!}{(n-1)! (n-1)!} \times (n-1)!
     \]
     \[
     a_n = \frac{(2n-2)!}{(n-1)!}
     \]
     \[
     a_n = (2n-2)!
     \]

The final answer is \( \boxed{ (2n-2)! } \).