A triangle is a technical analysis formation which implies with high probability, that a breakout or a breakdown is about to occur any time soon. There are three kinds of triangles. The ascending triangle implies a breakout, the descending triangle implies a breakdown and the symmetrical triangle implies both, depending on which direction price will move after breaking the formation. Classical technical analysis argues that an ascending or descending triangle should naturally break at about 2/3 of its length.
It implies the constant pressure of buyers who open long positions in every downward correction but progressively at a higher level (points 1, 2, 3, 4, 5, below). Therefore, they are constantly pushing prices higher until a breakout occurs. The upper line is just a horizontal resistance. We trade ascending triangles just like breakouts.
It impies the constant pressure of sellers that sell or open short positions in every upward correction but progressively at a lower level (points 1, 2, 3, below). Therefore, they are constantly pushing prices lower until a breakdown occurs. The lower line is just a horizontal support. We trade descending triangles just like breakdowns.
In symmetrical triangles, unlike ascending or descending, buyers and sellers have equal power until near the apex, where a breakout or a breakdown occurs. Then we trade them almost like breakouts or breakdowns. The upper line is a trendline resistance, whereas the lower line is a trendline support.
The only difference is that after the break of the formation, we expect a lengthy and violent movement, so we can set wider profit targets than the first resistance or support implies.