_default
Type: modifier
The global _default tag filter outputs when the tag produces an zero-length, empty string. 
Some tags have a default attribute, but the _default modifier works globally for all tags.
Example
Add default keywords:
<$mt:EntryKeywords _default="default keywords go here"$>
If an author leaves a entry title empty, you can force it to have a title:
<$mt:EntryTitle _default="Untitled"$>


