RE: What are the differences between the SPX and the SPY to an Options spread trader?

Thank you for the compliment.
As for your question?: Notional size can be used to describe or compare two trading vehicles of unequal price but proportionate returns. In this case one contract of the SPY costs 1/10 the price of one contract of SPX, but the premium is 1/10 of the size of a SPX contract. So you would need to buy 10 SPY contracts to get the same premium as one contract of SPX. At first this seems like an insignificant difference, but since brokers charge the same price per contract, a trade on the SPY would cost you $20.00 in fees versus $2.00 on the SPX.
Thanks for the question.
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