What is the next letter?
$$\text{P}:\text{Q}::\text{T}:\underline{}$$

1. Examine the relationship between the first pair $\text{P}:\text{Q}$:
* In alphabetical order, $\text{Q}$ comes immediately after $\text{P}$.
* Positionally: $\text{Q}=\text{P}+1$.

2. Apply the identical rule to the second pair starting with $\text{T}$:
* $\text{T}+1=\text{U}$.

3. Therefore, the missing letter is $\text{U}$.