[Zope] getProperty title and id

Hi, I just want to why <dtml-var "xx.getProperty('title')"> will display the object title and and <dtml-var "xx.getProperty('id')"> will dispaly none. <dtml-let xx=" _.getitem('id')"> <dtml-var "xx.getProperty('title')"> <dtml-var "xx.getProperty('id')"> </dtml-let> Thanks.