The selected item is displayed in the control, and there's a button with a down arrow on it that displays a menu with all of the available choices. This keeps the control compact and from taking up too much screen space.
NoteSome combo boxes allow you to enter text for the selected item (which, incidentally, is where the combo box gets its name), but I didn't use any of those in the program, so I won't be covering them here.
There are two slightly different ways to use the mouse to pick an item. You can either click on the combo box and then click on an item in the menu, or you can press the mouse button down on the combo box, drag the pointer to the item you want, and release the button. A combo box is a type of menu, but it is used for setting an option rather than executing a command.
You can also use the keyboard to select items. The up and down arrow keys will move the current selection up and down through the choices. You can also press the first letter of an item to select that item. If there is more than one item with the same first letter, pressing the key repeatedly will step through all the items that begin with that letter.
In the TX81Z Programmer, I have modified the combo boxes (as well as many other controls) to support keyboard navigation with the arrow keys, so the up and down arrows keys do not work on them. The functionality of these keys has been replaced with the Insert and Delete keys (see Keyboard Navigation for more about this).