[Zope] Where does "field_" prefix come from in Formulator fields?

[Skip Montanaro]

It appears that when expanding a Formulator form field a "field_" prefix
gets tacked onto the field name.  For example, one field I have is named
"postal_code".  When this

  <td tal:content="structure here/form/postal_code/render">item</td>

is rendered it expands to

  <td><input size="10" value="" type="text" name="field_postal_code"  /></td>

although I named to the object "postal_code", not "field_postal_code".
Where does "field_" come from and how can I get rid of it?

Thanks,

-- 
Skip Montanaro - skip@pobox.com
http://www.mojam.com/
http://www.musi-cal.com/