Voting within Congress can take many forms, including systems using lights and bells and electronic voting. Both houses use voice voting to decide most matters in which members shout "aye" or "no" and the presiding officer announces the result. The Constitution, however, requires a recorded vote if demanded by one-fifth of the members present or when voting to override a presidential veto. If the voice vote is unclear or if the matter is controversial, a recorded vote usually happens. The Senate uses roll-call voting, in which a clerk calls out the names of all the senators, each senator stating "aye" or "no" when their name is announced. In the Senate, the Vice President may cast the tie-breaking vote if present when the senators are equally divided.