Why are type variables for generics commonly declared as a single letter (e.g.: <T>)?

Question

For every example in the TypeScript generics documentation and in most type definition files I've came across, type variables are declared as a single letter, usually <T>.

Example:

function identity<T>(arg: T): T {
    return arg;
}

Questions:

• why are type variables for generics commonly declared as a single letter?
  • what are the reasons for doing this?
  • does it have a purpose other than convention and brevity?
• what are the pros and cons of using of more descriptive names?

Answer 1

The Java docs for Generics Types has a really good paragraph about this - and it applies to all languages that supports type variables:

Type Parameter Naming Conventions

By convention, type parameter names are single, uppercase letters. This stands in sharp contrast to the variable naming conventions that you already know about, and with good reason: Without this convention, it would be difficult to tell the difference between a type variable and an ordinary class or interface name.

The most commonly used type parameter names are:

....

T - Type

Basically you want to be able to quickly recognize that something is a type variable, and naming the type variable T just became a standard code convention at some point.

Microsoft's docs on C++ and C# has this convention too.