It is common in mathematics to use subscripted variables. We use the underscore to represent subscripts. If we haven't assigned a value to x we may simply start using it as a subscripted variable. The subscripts can be anything.
i1 : x
o1 = x
o1 : Symbol
i2 : x_4
o2 = x
4
o2 : IndexedVariable
i3 : x_(2,3)
o3 = x
2,3
o3 : IndexedVariable
The .. operator knows what to do with subscripted variables.
i4 : x_10 .. x_20
o4 = (x , x , x , x , x , x , x , x , x , x , x )
10 11 12 13 14 15 16 17 18 19 20
o4 : Sequence
i5 : x_(1,1) .. x_(2,3)
o5 = (x , x , x , x , x , x )
1,1 1,2 1,3 2,1 2,2 2,3
o5 : Sequence