I Spy Strangers
Anyone who isn't an MP or a parliamentary official is known as a stranger, and historically were not allowed into Parliament to view debates. Nowadays the public can watch from the Gallery, although anyone causing a disturbance can be thrown out, or even taken into custody by the Serjeant at Arms.
Parliament still has the right to debate in private, and any MP wishing to exclude the public must shout "I Spy Strangers". The Speaker then puts forward the motion "That Strangers do now withdraw". If the motion is carried, the Galleries -including the Press Gallery- are cleared and the business continues in private.
An attempt to clear strangers is usually only employed as a device to disrupt or delay the House's proceedings.
