Joseph Stavitsky wrote:
Found it, it's the period key.
Yes, it's called the "literal subscript" as opposed to the vector or matrix subscript.
For someone new to Mathcad its very often a subject for confusion.
In prime, btw, the literal subscript is achieved by typing "Ctrl -" a bit awkward unfortunately. But the enhancement is that in contrary to MC15 you can switch back and forth between normal and subscripted characters - especially usefull for chemists, I guess.