That means the bond price has been bid up. IIRC, interest rates rise when there is less demand in buying bonds (thus trying to get buyers to get interested in buying the bonds). When there is high demand for bonds, the price to buy the bonds goes up, yet interest rates fall. When there is very high demand for bonds, the price could keep going up to the point where there is negative rates. People buy the bonds not necessarily to make money off the bond interest, they hold them as like a stock investment, the price of the bond could keep going up, so the bond holder can make some money depending on what price the buyer bought the bond. I have no clue where I learned this, probably watching CNBC world or something like that.
This system rewards solvent countries like Germany and punishes spend-thrifts like Greece.
The inverse relation between interest rates and bond prices may seem illogical at first but it makes sense upon closer examination.
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If you have a falling/weakening currency, you don't lose money investing in German Bonds with a strong euro. Same is true with a strengthening Yuan. There are many reasons to invest in negative rates.