438 Part IV: Additional Considerations 
Put price = Striking price + Call price - Stock price - Fixed cost 
Furthermore, if the stock is at the striking price, the formula reduces to: 
Put price = Call price - Fixed cost 
So, whenever the fixed cost, which is equal to the carrying charge less the dividends, 
is greater than zero (and it usually is), the put will sell for less than the call if a stock 
is at the striking price. Only in the case of a large-dividend-paying stock, when the 
fixed cost becomes negative (that is, it is not a cost, but a credit), does the reverse 
hold true. This is supportive evidence for statements made earlier that at-the-money 
calls sell for more than at-the-money puts, all other things being equal. The reader 
can see quite clearly that it has nothing to do with supply and demand for the puts 
and calls, a fallacy that is sometimes proffered. This same sort of analysis can be used 
to prove the broader statement that calls have a greater time value premium than 
puts do, except in the case of a large-dividend-paying stock. 
One final word of advice should be offered to the public customer. He may 
sometimes be able to find conversions or reversals, by using the simplistic formula, 
that appear to have profit potentials that exceed commission costs. Such positions do 
exist from time to time, but the rate of return to the public customer will almost 
assuredly be less than the short-term cost of money. If it were not, arbitrageurs would 
be onto the position very quickly. The public option trader may not actually be think­
ing in terms of comparing the profit potential of a position with what he could get by 
placing the money into a bank, but he must do so to convince himself that he cannot 
feasibly attempt conversion or reversal arbitrages. 
THE "INTEREST PLAY" 
In the preceding discussion of reversal arbitrage, it is apparent that a substantial por­
tion of the arbitrageur's profits may be due to the interest earned on the credit of the 
position. Another type of position is used by many arbitrageurs to take advantage of 
this interest earned. The arbitrageur sells the underlying stock short and simultane­
ously buys an in-the-money call that is trading slightly over parity. The actual amount 
over parity that the arbitrageur can afford to pay for the call is determined by the 
interest that he will earn from his short sale and the dividend payout before expira­
tion. He does not use a put in this type of position. In fact, this "interest play" strat­
egy is merely a reversal arbitrage without the short put. This slight variation has a 
residual benefit for the arbitrageur: If the underlying stock should drop dramatically 
in price, he could make large profits because he is short the underlying stock. In any 
case, he will make his interest credit less the amount of time value premium paid for 
the call less any dividends lost.