Puts and calls are obviously different option
types. However, there is a mathematical relationship between calls and puts
when the put and call expire at the same time, have the same strike
price, and are on the same underlying asset.
Because of that relationship, there is more
than one way to build any option position -- and that means that
there are equivalent positions (i.e., positions with identical profit/loss
profiles) -- even though the positions appear to be very different.
Although you can survive by avoiding the small
amount of homework involved in understanding this concept, but it does mean
that you will occasionally be leaving money of the table for no good
reason. Isn't that why you are trading? To make money?
Traders own positions with an
expectation of earning a profit when the markets behave. If you can own a
different position that results in the same profit (or loss), but which
requires paying less in commissions, wouldn't that be preferable?
Infrequently you may discover that the markets
are temporarily inefficient (it won't last long), and that one of
the equivalent positions is available at a slightly better (perhaps $0.05)
price than its equivalent. If you spot that difference, you can own the
position with that $0.05 discount.
The
basic equation is often referred to as put-call parity.
You can find more details here.
For the purpose of introducing this topic, the effect of interest rates
is ignored.
Put-call parity describes the relationship
between calls, puts, and the underlying asset.
Owning one call option and selling one put
option on the same underlying asset (with the same strike price and expiration
date) is equivalent to owning 100 shares of stock. Thus,
S = C – P
Where S = 100 shares of stock; C = one
call option ; P = one put option
Simple
proof: Consider a position with one long
call and one short put. When expiration arrives, if the call option is in the
money, you will exercise the call and own 100 shares. If the
put option is in the
money, your account will be assigned an exercise notice and you must buy 100 shares. In
either case, you own stock.
NOTE: If the stock is exactly at
the money when expiration arrives, you are in a quandary. You don’t know
whether the put owner will exercise and therefore, you do
not know what to do with your call. The best solution is to buy the
put to cancel any obligations. It should not cost more than $0.05. Next,
if you do want to own stock, exercise the call option. If not, allow the
call to expire worthless. By covering the short put, you are in control.