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Steem Dollars will simply maintain there value at roughly 1 USdollar each regardless of the price of Steem. If the Steem price is $1, you can expect to get 1 Steem from a Steem Dollar. If the price of Steem is $0.10 then you can expect to got 10 Steem from a Steem Dollar.

For Steemdollar you also get 10 % interest per year. Not bad in an age of negative interest in the fiat currency system.