Drop-Down List
Implementation: ComboBox
A drop-down list is a control that displays a list of choices on click and allows selecting one option.

Use when a single option should be selected.

When multiple selections are possible, use checkboxes.
The possible choices are objects or states.

When options are actions, use a menu or a split button.
The number of options is more than four.

When there are four options or fewer, use radio buttons.
The screen space is limited, so there is not enough room for radio buttons.

The default value is recommended for most users. A drop-down is a good way to hide unpopular alternatives.
In the example bellow a drop-down is used because Subpixels is the most popular choice. Using radio-buttons puts all options on the same level of importance or popularity.

If a control combines various UI elements as one setting, use a drop-down even if there are four options or fewer:

If there are four options or fewer, use radio buttons.
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If multiple selection is possible, use checkboxes.
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For a single selection use a drop-down.
If there is a list of actions, use a menu or a split button.
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For a list of objects or states, use a drop-down.
If users may need to enter a value not currently in the list, use a combo box.
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If a drop-down appears on a toolbar, use toolbar drop-down instead.
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Follow the rules for the input field.

Select the most likely or the safest value by default. In the xample bellow the safest value is Ask, the behavior will not be unexpected to users:

Do not use an empty value as the default value. If no values are added to a drop-down yet, replace it with a button to add values.
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Use sentence-style capitalization for each menu item.
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Avoid repeating words in drop-down list items. Move repeating words to the label or after the drop-down.
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Sort items in one of the following orders:
![]() | Logical order, for example, in a spatial relationship. |
![]() | Alphabetical or numeric order if the options are equivalent to make it easier to find items. |
![]() | Place the most common options first. If there are more than 10 options, separate the most popular options with a line. Sort the remaining items in alphabetical or numeric order. |
Group related items using separators. Add group headers if possible.
With dividers ![]() | With a divider and group headers ![]() |
If there are actions related to the drop-down, put them under the gear icon on the right:

note
Meta-option is a special choice that controls how selections behave rather than being a specific value.
Enclose meta-options in pointy brackets and place them at the beginning of the list when they're important or popular options. In other cases, place them at the end of the list.

Do not assign special meanings to numeric or string values, use meta-options instead. It can be unclear that either 0 or an empty string are used to disable an option.
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Replace the <None> option with a checkbox if it doesn't make the layout more complex.
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![]() | Add icons to the values if they're well-known. |
![]() | Use font formatting for a list of fonts. It will help users make a choice. |
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Submenus are not supported for the drop-down list. Use a tree view instead

A drop-down menu opens on clicking the drop-down button anywhere or pressing the Down key when the drop-down is focused.
The menu opens down by default. If there is not enough space, the menu opens up.
When the menu opens, the default option (displayed when the menu is closed) is selected.
Selection in the menu is moved by pressing Up and Down keys when an item is hovered. Selection is moved as well when hovering the cursor over an item but it doesn't update the value.
The menu remains opened until the user selects an item, clicks outside the menu, presses the Esc key, or switches to another app.
A dropdown should work with a single mouse click:
Clicking to open a drop-down.
Hover over an item to select it.
Release the mouse to confirm the selection and close the dropdown.
Follow the labeled input controls guideline.
Thanks for your feedback!