When purchasing put options, you are expecting the price of the underlying security to go down over time (so, you're bearish on the stock). For example, if you are purchasing a put option on the S&P 500 index with a current value of $2,100 per share, you are being bearish about the stock market and are assuming the S&P 500 will decline in value over a given period of time (maybe to sit at $1,700). In this case, because you purchased the put option when the index was at $2,100 per share (assuming the strike price was at or in the money), you would be able to sell the option at that same price (not the new, lower price). This would equal a nice "cha-ching" for you as an investor.
Based on data from IHS Markit for SEC Rule 605 eligible orders executed at Fidelity between April 1, 2018 and March 31, 20198. The comparison is based on an analysis of price statistics that include all SEC Rule 605 eligible market and marketable limit orders of 100-499 shares for the 100 share figure and 100–1,999 shares for the 1,000 share figure. For both the Fidelity and Industry savings per order figures used in the example, the figures are calculated by taking the average savings per share for the eligible trades within the respective order size range and multiplying each by either 100 or 1000, for consistency purposes. Fidelity's average retail order size for SEC Rule 605 eligible orders (100 -1,999 shares) and (100–9,999 shares) during this time period was 430 and 842 shares, respectively. The average retail order size for the Industry for the same shares ranges and time period was 228 and 333 shares, respectively. Price improvement examples are based on averages and any price improvement amounts related to your trades will depend on the particulars of your specific trade.
For example, if you bought a long call option (remember, a call option is a contract that gives you the right to buy shares later on) for 100 shares of Microsoft stock at $110 per share for December 1, you would have the right to buy 100 shares of that stock at $110 per share regardless of if the stock price changed or not by December 1. For this long call option, you would be expecting the price of Microsoft to increase, thereby letting you reap the profits when you are able to buy it at a cheaper cost than its market value. However, if you decide not to exercise that right to buy the shares, you would only be losing the premium you paid for the option since you aren't obligated to buy any shares. 

The price at which you agree to buy the underlying security via the option is called the "strike price," and the fee you pay for buying that option contract is called the "premium." When determining the strike price, you are betting that the asset (typically a stock) will go up or down in price. The price you are paying for that bet is the premium, which is a percentage of the value of that asset. 
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