This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

Typing Subscripts

Typing x_i

Type the symbol that will have the subscript.
Type .
Click for copyable input
Create a subscript by pressing the Control key and the - key at the same time.
Press Ctrl+- or Ctrl+_.
Click for copyable input
Type the subscript into the placeholder (which is already selected).
Type .
Click for copyable input
Move the insertion point out of the subscript position up to the baseline by pressing the Control key and the Space bar at the same time.
Press Ctrl+Space.
Click for copyable input

Typing x_(i j)

Type the symbol that will have the subscript.
Type .
Click for copyable input
Create a subscript by pressing the Control key and the - key at the same time.
Press Ctrl+- or Ctrl+_.
Click for copyable input
Type the subscript into the placeholder (which is already selected).
Type .
Click for copyable input
Press the Space bar and then type .
Press Space and type .
Click for copyable input
*To avoid multiplying the two subscripts, separate them with an invisible comma. To type the alias for an invisible comma, press the Escape key before and after typing a comma.
Press Esc, type a comma, press Esc, and type .
Click for copyable input
Move the insertion point out of the subscript position up to the baseline by pressing the Control key and the Space bar at the same time.
Press Ctrl+Space.
Click for copyable input

Typing x_(2⁣3)

Type the symbol that will have the subscript.
Type .
Click for copyable input
Create a subscript by pressing the Control key and the - key at the same time.
Press Ctrl+- or Ctrl+_.
Click for copyable input
Type the subscript into the placeholder (which is already selected).
Type .
Click for copyable input
To avoid multiplying the two subscripts, separate them with an invisible comma. To type the alias for an invisible comma, press the Escape key before and after typing a comma.
Press Esc, type a comma, press Esc, and type .
Click for copyable input
Move the insertion point out of the subscript position up to the baseline by pressing the Control key and the Space bar at the same time.
Press Ctrl+Space.
Click for copyable input

Typing x_(n_i)

The first three steps create and fill in the subscript and its subscript.
Type , then press Ctrl+_.
Click for copyable input
Type , then press Ctrl+_.
Click for copyable input
Type .
Click for copyable input
Press the Control key and the Space bar at the same time twice to get up to the baseline.
Press Ctrl+Space twice.
Click for copyable input
Click for copyable input