Why is it customary in functional languages ​​to use the xs abbreviation for lists?

Question

Functional programming

Sum of elements on OCaml (Wikipedia).

let rec sum xs =
  match xs with
    | []       -> 0
    | x :: xs' -> x + sum xs'

Same thing on scala (Stackoverflow).

def sum(xs: List[Int]): Int = {
  xs match {
    // if there is an element, add it to the sum of the tail
    case x :: tail => x + sum(tail) 

    // if there are no elements, then the sum is 0
    case Nil => 0 
  }
}

Answer 1

Most likely because "x" is an element and "s" is plural. A list is a set of elements. Something like item -> items