wall street. chief washington correspondent james rosen tonight explains what china did and what it means. >> reporter: from the people's bank of china came the surprise overnight announcement that the chinese government is lowering the guidance rate for its currency, the yuon by 4.9%. largest one-day devaluation by china in two decade. it caused shock contractions at all major stock markets. japan's nikkei tumbled. the german dax and our own dow jones industrial average plunged 212 points. u.s. officials spoke guardedly, but once again urged china to move a more market-based exchange rate. >> we have pressed china to continue financial rear forms, and while we want to see additional economic reforms we believe that are needed, but we have seen progress. >> reporter: china's move came after its release this weekend of data showing that exports from the world's second largest