combobox - Guidelines for representing sentinel values within a list

I'm curious if anybody can point me to a list of guidelines (preferable regarding Windows Development UI design) that may help layout rules which to follow when representing sentinel or special values within a list. An example of such a value could be a none or all option within a ListBox or ComboBox control.

Answer

Looks like the Windows 7 UX Guide has some pointers specific to special values which they refer to as meta-options. The same recommendations are also made for meta-options wihtin ComboBoxes.

• Place options that represent All or None at the beginning of the list, regardless of sort order of the remaining items.


• Enclose meta-options in parentheses.



• Don't have blank list items—use meta-options instead. Users don't know how to interpret blank items, whereas the meaning of meta-options is explicit.

It's interesting that the options representing all are not placed in parenthesis like the none option is in their examples. Anybody have any thoughts on this?