Why are List and String identifiers named "xs" (in Scala and other languages)?

Question

A lot of sample Scala code contains Strings and Collections named "xs". Why xs?

Examples:

var xs = List(1,2,3)
val xs = "abc"

Answer 1

Basically it's a naming convention that originated in LISP. The rationale behind it is that:

1. X is a common placeholder name.
2. XS is pronounced X'es, i.e "many X".