Some examples: 1. Futures are contracts agreeing to buy or sell some underlying asset at a future date (or exchange cash as though you did). If I am long a fut

Some examples: 1. Futures are contracts agreeing to buy or sell some underlying asset at a future date (or exchange cash as though you did). If I am long a future someone is by definition short that future. Therefore the total net exposure is always zero. It makes no sense to add my long future to your short future and decide that the total exposure is 2. 2. Swaps are contracts where we agree to exchange cashflows based on some reference amount. The most common types of swaps are interest rate swaps and an example might be a fixed-for-floating swap where you agree to pay me fixed interest (say 4%) and I agree to pay you floating interest (say 3 mounth USD LIBOR plus 200bps). The notional of a swap is the amount we use for the calculation of this interest payment. It really isn't an amount that's used for anything else - you don't owe me the notional and neither do I owe you that. It's just a multiplier we use when we transfer the net difference between the two rates. It therefore makes no sense whatsoever to add up the notional of swaps contracts. Also since we exchange the net difference the notional can be pretty large and the net cash payments pretty small. 3. Credit default swaps are similar to IR swaps except one of the legs of the swap pays out based on certain "credit events" (typically defaults and/or downgrades) on an underlying "reference obligation" (usually a corporate or sovereign bond, but can be an ABS or probably some other stuff). Since different CDSs are based on different underlying reference obligations, adding up notionals of different CDSs makes as much sense as if I say I have 10 dollars, 2 matchsticks, a ball of fluff, a ferarri and a dog so you say I have 15 things. Well I suppose so but it doesn't really mean anything since the things themselves are very different in nature.