 The symbols “\(\amp\)”, “\(\vee\)”, and “\(\rightarrow\)” are called “binary connectives”, since they serve to “connect” two formulas into one. Some authors introduce \((\theta \leftrightarrow \psi)\) as an abbreviation of \(((\theta \rightarrow \psi) \amp(\psi \rightarrow \theta))\). The symbol “\(\leftrightarrow\)” is an analogue of the locution “if and only if”.