What exactly are mezzanine securities?

Mezzanine originally refers to an intermediate floor between the first and second floors of a building, and in finance it refers to securities that sit between bonds and shares.

Korea Financial Investment Association (KOFIA) guidelines define mezzanine securities as all securities in an intermediate form between shares and bonds, such as convertible bonds (CBs), exchangeable bonds (EBs), bonds with warrants (BWs), convertible preferred shares and redeemable convertible preferred shares (RCPS). What they have in common is that they start out as bonds or preferred shares but carry a right to turn into, or receive, shares under set conditions.