Occasionally companies use preferred shares as means of preventing hostile takeovers, creating preferred shares with a poison pill (or forced-exchange or conversion features) which are exercised upon a change in control. Some corporations contain provisions in their charters authorizing the issuance of preferred stock whose terms and conditions may be determined by the board of directors when issued. These "blank checks" are often used as a takeover defense; they may be assigned very high liquidation value (which must be redeemed in the event of a change of control), or may have great super-voting powers.