This could be the key to the dollar's next move

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The dollar index is closing in on its March highs, a possible springboard to 12-year and even all-time highs.

With the Fed positioning for a December rate hike, the dollar index, which tracks the dollar's performance against six currencies, has risen more than 2.2 percent in the past week. It was trading at about 99.4 Tuesday.

"What everybody's looking at is 100.39 set in mid-March, a percent higher than where we're currently trading. Above there are levels we haven't seen in a long time. The last time it was above 100 was back in 2003," said Marc Chandler, head of currency strategy at Brown Brothers Harriman.