A combo box combines a text field with a pull-down button in a single control. The user can enter a custom value into the field or click the button to choose from a list of predefined values. When the user enters a custom value, it’s not added to the list of choices.
Populate the field with a meaningful default value from the list. Although the field can be empty by default, it’s best when the default value alludes to the hidden choices. The default value doesn’t have to be the first item in the list.
Use an introductory label to let the user know what types of items to expect. Generally, labels should use title-style capitalization and end with a colon. For related guidance, see Labels.
Provide relevant choices. Users appreciate being able to enter a custom value, but also appreciate the convenience of choosing from a list of the most likely choices.
Offer standalone choices. Combo boxes don’t allow multiple selections; the user can only choose a single value from the list.
Make sure list items aren’t wider than the text field. If an item is too wide, it can be truncated and hard to read.