As this happens the price changes of the call option begin to change in-line with changes in the underlying stock. On the right you will notice that as the stock price rises the call options increase in value. Now, at either end of the graph each option will either be in or out of the money. ATM options are therefore said to be '50 Delta'. When the stock is at $25 both options are at-the-money and will change in price by the same amount as the underlying moves, which is +/- 0.50. The first thing to notice is that option prices do not change in a linear movement versus the underlying the magnitude of the option price change depends on the options' 'moneyness'. The corresponding call and put options for the x-axis stock prices are plotted above call in blue and put in red. The dotted line represents the price 'change' for the underlying with the actual price of the stock on the horizontal axis. The chart compares the movement of an underlying versus the option prices at each underlying level for both a call and put option with a $25 strike price. This is why the delta is important it takes much of the guess work out of the expected price movement of the option. Whether it is a call or put, the proximity of the strike to the underlying price, volatility, interest rates and time to expiry.

There are many factors that will affect the price that an option will change by e.g. As the price of the underlying stock fluctuates, the prices of the options will also change but not by the same magnitude or even necessarily in the same direction.

Base delta zero vs glassing