The best-performing stocks in the Dow Jones industrial Average so far in 2017? It's Boeing Co. (BA) - Get Report by a mile.

Boeing is up a staggering 52% on a price basis since the calendar flipped to January, leaving the rest of the bellwether stock market index in its dust. Even number-two Dow performer Apple Inc. (AAPL) - Get Report is trailing Boeing's by nearly 15 percentage points. That's a massive performance gap -- and it could be about to widen.

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That's because, as I write, Boeing is on the verge of another breakout. To figure out how to trade it, we're turning to the charts for a technical look:

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This isn't the first time I've talked about Boeing's technical trajectory. Since that was published less than a month ago, Boeing has added nearly 12% to its share price -- tacking about $15 billion in market value onto its capitalization. But shares aren't looking fatigued here. Quite the opposite.

Boeing is currently forming a pretty textbook example of an ascending triangle pattern, a bullish continuation setup that signals more upside ahead. The pattern is formed by horizontal resistance up above shares (at $240 in Boeing's case), with uptrending support to the downside.

Basically, as shares of Boeing have pinballed in between those two technically significant price levels, this stock has been getting squeezed closer and closer to a breakout through its $240 price ceiling. When that happens, we've got a brand new buy signal in shares.

What makes that $240 level so important for this stock? It all boils down to buyers and sellers.

Price patterns, like this ascending triangle setup, are a good quick way to identify what's going on in the price action, but they're not the ultimate reason shares look attractive here. Instead, the "why" is driven by basic supply and demand for Boeing's shares themselves.

The $240 resistance level is a price where there has been an excess of supply of shares; in other words, it's a spot where sellers have been more eager to step in and take gains than buyers have been to buy since the beginning of August. That's what makes a breakout above $240 so significant -- the move means that buyers are finally strong enough to absorb all of the excess supply above that price level.

Relative strength adds some extra confirmation that Boeing's outperformance continues to be alive and well as the summer months wind down. As long as relative strength continues making higher lows in the long-run, Boeing is statistically predisposed to keep on outperforming.

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This article is commentary by an independent contributor. At the time of publication, the author was long AAPL.