Combo boxes (also known as a dropdown box) offer the user a choice of two or more mutually exclusive options. Their disadvantage compared to related controls like radio buttons or lists is that the options are not visible without further interaction.

[edit] Guidelines

• For 3 or fewer options, use a set of radio buttons.
• For 4 to 10 options, use a combo box instead of radio buttons.
• For more than 10 options, use a list.
• Label the combo box with a descriptive label to the left of the combo box.
• Use sentence style capitalization for the label and the options.
• If activating a choice affects the appearance or the enabled state of other controls, place them next to the combo box or below the combo box with a space indentation.
• Make sure the items are easily accessible via keyboard by moving distinctive letters to the beginning of each option. For example, in a list of countries on continents, write Germany (Europe) instead of Europe/Germany.
• Don't initiate an action when the user selects an item from a drop-down list.
• Don't forget to create a buddy relation so access keys are assigned.

Controls that show the options without further user interaction are preferable, except for the following cases:

• the list of options may change over time,
• the contents are obvious from the label and the one selected item, for example Month and January
• the combo box is part of a related sequence of controls. For example, to set a reminder to ring 5 hours or minutes before or after an event.