+**index.header.<prefix>** **[STORED IN DATABASE]**
+ Define the query prefix <prefix>, based on a mail header. For
+ example ``index.header.List=List-Id`` will add a probabilistic
+ prefix ``List:`` that searches the ``List-Id`` field. User
+ defined prefixes must not start with 'a'...'z'; in particular
+ adding a prefix with same name as a predefined prefix is not
+ supported. See **notmuch-search-terms(7)** for a list of existing
+ prefixes, and an explanation of probabilistic prefixes.
+