This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

Opener

Opener[x]
represents an opener with setting x, displayed as when x is True and when x is False.
Opener[Dynamic[x]]
takes the setting to be the dynamically updated current value of x, with the value of x being toggled if the opener is clicked.
New in 6