The "classical" triangle has at least 5 waves, and the breakthrough happens at about 2/3 of the horizontal size of a triangle. It is not enough for the price to touch the side of a triangle, the price bar must close outside the triangle, otherwice we might have a false signal.

During the uptrend the triangle will more likely produce a false signal, especially if the breakthrough happened too close to the end of a triangle.

Symmetrical triangles

This pattern can be considered as a sign that the market is "uncertain" in which direction to move. Both buyers and sellers are pushing the price towards some middle value. As it happens the volume usually is decreasing as everybody is waiting for the price to break out of the triangle. The moment it happens, the volume is usually increasing - due to psychological reasons.

It seems that the triangle USUALLY does not change the trend - if the price was going up, then it is most probable that it will break UP from the triangle and vice versa.

Ascending triangles

This pattern is a variation of the Symmetrical triangle. Think of it in terms of "increasing pressure up and constant pressure down". The pattern will most probably be resolved UP.

As the pattern is forming, the volume is deminishing, and when the breakthrough occures, the pattern is usually expanding. It is a useful (but not mandatory) confirmation.

The price projection equals the maximim height of the triangle.

Descending triangles.

This pattern is a variation of the Symmetrical triangle. Think of it in terms of "increasing pressure down and constant pressure up". The pattern will most probably be resolved DOWN.

The length of the patter should be somewhere between few weeks and few months.

As the pattern is forming, the volume is deminishing, and when the breakthrough occures, the pattern is usually expanding. It is a useful (but not mandatory) confirmation.

The price projection equals the maximim height of the triangle, measured from the resistance breakout point.

Double top and Double bottom

This is a reversal pattern that forms after an uptrend. It consists of two peaks.

The important confirmation signal occures when the support line is broken after the second peak.

The decline after the first peak is somewhere between 10 and 20 %.
The duration of the pattern is somewhere between few weeks and few months.
The size of peaks is nearly equal.

The decline from the second peak may contain gaps and the volume should expand.
The support line should be broken, for the pattern to be clearly identified.
The price target equals the size of the peak.

Head and shoulders

When the stock is going up, we can use the concept of a SUPPORT line, the line below the price, that price is constantly testing but cannot cross. Then when the trend is changing, the support line is broken AND for a short period of time the chart can be considered horizontal - the temporary support line is called a neckline.

Then the price is trying to reach the previous support line (and to restore the trend) but failing (right shoulder). This is the first "sell" signal. The second one is happening when the right side of the right shoulder penetrates the neckline - this sell signal is much stronger. Again as always, a lot of this strength the pattern gets from the fact that people know about it, believe in it and when it happens - they begin selling, therefore pushing the price down.

The volume is very important both for the head and shoulder formation, and for the reversed (bottom) head and shoulder. In both cases, the volume must expand when the resistance (support) is finally broken. Volume can be measured by the corresponding indicator (OBV, Chaikin Money Flow). Basically, the volume moves in the opposite direction to the price: the price is going up (to the top of the head) - the volume declines and vice versa.

The price target equals the distance between the neckline and top of the head.

Wedges

Unlike with the triangles, both upper and lower edges are either going up (bearish formation) or down

The volume is not that important for the rising wedge, but it is critical for the falling wedge. The volume should expand to confirm the break of a resistance.

Flags

Flags are (sort of) variations of a triangles. They can be explained using the same logic and they predict the same thing. The pattern is considered part of the trend, which means that on the uptrend it is a sign that price will continue to increase and on the downtrend it is a sign that the price will fall.

Rectangles

Same as flags. A temporary slowdown in the trend that usually wouldn't change it. They are usually preceded by a sharp advance or decline with heavy volume.

For other approaches, please read Encyclopedia of Chart Patterns by Thomas N. Bulkowski