Occasionally, good interface design means bending the rules somewhat.
According to Apple's interface guidelines, radio buttons should be used when you want to give the user two or more mutually exclusive choices. You can see this in the Search for Words/Verses radio buttons in the Search window. You can pick one or the other, but not both.
If you want to give the user a large number of mutually exclusive choices, your best bet is to use a pop-up menu. The pop-up menu gives you a space-saving way to list numerous options. When the user selects an item from the menu, that item is displayed in the pop-up menu itself. If you click and hold on the pop-up menu to see all the items, the selected item will have a check mark beside it.
If you want to enable the user to select multiple items, you're supposed to use checkboxes. Checkboxes are similar to radio buttons in that each option is visible at the same time, but where only one radio button can be activated at a time, checkboxes can be used to select multiple options in combination.
Now, the problem with checkboxes is that they take up a lot of space, and if you have a bunch of different choices, you end up with so much visual clutter it can be hard to find the boxes you want to check.
One place where users need the ability to enter a variety of options in combination with each other is in the entry of Greek and Hebrew grammatical tags. For example, I may want to find verbs in the first or second person, the aorist, imperfect, or perfect tenses, the middle voice, and the imperative or infinitive moods. That's a lot of different options to select.
Most programs which offer an interface for selecting these kinds of options present their users with a dialog box peppered with checkboxes for each individual grammatical characteristic. Technically, they're using the correct interface element—the one which allows for multiple choices in combination with each other—but the unfortunate side effect is a visually overwhelming interface.
In Accordance, the tag entry dialog box for each part of speech uses a simple series of pop-up menus to present the user with all the available choices:
But remember, a pop-up menu is designed to let the user select just one option, which gets displayed in the pop-up menu itself. So what if you want to select more than one option in each grammatical category?
Here's where we bend the rules a bit. To select more than one option from each pop-up (such as first and second from the Person pop-up), just hold down the shift key while selecting the options you want. The most recent tag you selected will be displayed in the pop-up menu, and any additional tags will be listed to the right of the pop-up. Like so:
As I said, we're bending the rules a bit, but the result is an interface which avoids the visual confusion of innumerable checkboxes while still displaying every option the user selects.
You can do the same thing with the style pop-up menus of the various Text Display dialogs. For example, if you want your Bible text References to appear in bold italic, simply hold the shift key down while selecting both bold and italic from the pop-up menu. Some sample text to the right of the dialog will display all the currently selected styles.
You can also shift-select multiple layers in the Map window to overlay, for example, all three of Paul's Missionary Journeys at once. When you do that, a plus appears in the pop-up beside the first selected layer to indicate that there are additional layers selected. If you click and hold the pop-up menu, a check mark will appear beside all the selected layers.
In every case, when we enable pop-up menus to be used to select multiple items, we offer visual feedback that more than one option has been selected. The result is an interface which is clean and simple, flexible enough to allow for multiple choices, and careful to keep the additional choices visible.