field type for True/False/Empty I have a field on a content type which is an optional override for another field. I want the value to only be either 0, 1, or empty, the last meaning "no override". What field type should I use for this? I was looking at a select list for a good user experience, but I was unable to get empty as a value in the list. 

I also need another field that can only have values "img", "txt" or "". How should I accomplish this? I want to avoid having users type in "img" or "txt" in case they misspell it.
Select lists have always the option None if they are not set as Required. That said, you can create a List (Text) field  with select list widget and set only two options, 0 and 1, and you will get the None option automatically.