Tri-state checkbox.
Shows that some, all, or none of a group are selected.
See it in context
What it is
The mixed or indeterminate state summarizes a partially selected collection. A parent checkbox can display a dash while some child items are checked and others are not.
When to use it
Keep the mixed state synchronized with child selections and define what parent activation selects.
Know the difference
Mixed is not a third independent answer; it commonly summarizes the state of multiple child choices.
Across platforms
Web · Windows · Linux
Expose checked, mixed, or pressed state using the corresponding native control or accessibility semantics. Space normally changes the focused control.
iOS · iPadOS · Android · macOS
Use the native selection control appropriate to immediate settings or form choices. A switch and a checkmark can express different interaction models.
Go to the source
Primary documentation for the pattern and its platform conventions.