HTML <sub> Tag

H<sub>2</sub>O

The above example demonstrates usage of the <sub> element.

The <sub> element represents subscript. A subscript is often used in mathematical expressions.

The HTML5 specification states that <sub> element must only be used only to mark up typographical conventions with specific meanings, not for typographical presentation for presentation's sake.

Also see the <sup> element for marking up superscripts.