Option is a child type of Tradable that represents a financial contract that grants its holder the right - but not the obligation - to enter into a prespecified contract, referred as the "underlying" contract. This voluntary act of entering into the underlying contract is called "exercising the option". An option may be a call or a put depending on whether its holder enters into the underlying contract on a long or short position respectively, when the option is exercised. Furthermore option exercising is generally allowed only once on a date that is chosen by the holder out of a preagreed set of future dates, called "exercise dates". The simplest case is that of a so called "vanilla european option", where there is only one possible "exercise date", meaning the holder must decide on that date whether or not to exercise the option.