Details and Options

UnderscriptBox is the low-level representation of Underscript. Except for low-level notebook expression manipulation, UnderscriptBox should not need to be used directly.
Inside \(…\) UnderscriptBox[x,y] can be input as x\+y.
In a notebook, an UnderscriptBox can be created using . moves out of the underscript position.
The following options can be given:

BaseStyle	{}	the style to use
DiacriticalPositioning	Automatic	where to draw diacritical marks
LimitsPositioning	Automatic	whether to position scripts in a way conventional for limits
MultilineFunction	Automatic	how to handle scripts so long that linebreaking may be necessary

In StandardForm, UnderscriptBox[x,y] is interpreted on input as Underscript[x,y].
UnderscriptBox[x,"_"] is interpreted as UnderBar[x] or .
The baseline of UnderscriptBox[x,y] is taken to be the baseline of x.
UnderscriptBox[x,y] is usually output with y in a smaller font than x.
With the option setting LimitsPositioning->True, y is placed in an underscript position when the whole UnderscriptBox is displayed larger, and in a subscript position when it is displayed smaller.
In StandardForm, explicit UnderscriptBox objects are output literally. You can use DisplayForm to see the display form of such objects.

Examples

Display a letter with an underscript:

An UnderscriptBox form is typically interpreted as an Underscript expression:

The standard formatting for Underscript uses UnderscriptBox:

The formatted result looks like this:

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