See the following website:
http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=c31c5cdc2c463110VgnVCM1000004718190aRCRD&vgnextchannel=fe529c7755cb9010VgnVCM10000045f3d6a1RCRD
http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=eb7b5cdc2c463110VgnVCM1000004718190aRCRD&vgnextchannel=fe529c7755cb9010VgnVCM10000045f3d6a1RCRD