The line arcs downwards so it is easily distinguishable from other lines around the same area. What the head line tells you about yourself is the fortitude of your intellect. The different proposed methods for reading the arc of the head line will tell you of the strength of your mental capacity, but there are two different ways to read the head line it in regards to timing. For example, if you were trying to get a general sense of how your intelligence has progressed since you were born; the segments of the line can be divided up three ways.
The first segment will tell you what your level of intelligence was like in your youth. The second segment will tell you want your intelligence was like into adulthood, or what it will be like if you are younger. Finally, the third segment will tell you what will happen to your intelligence in your old age. The other method of reading the head line in the form of a time line is to simply take the beginning of the line as the place where you are at in life currently, and see the progression of your intelligence throughout the years in the arcs and divots of the line throughout its progression.
Determining what the line means itself is relatively intuitive. If you have a head line that wavers like the lines on a lie detector test, this is indicative of your short attention span. This could also mean that you are a scatterbrained person, constantly jumping from one thing to the next mentally and having trouble focusing on certain things. This does not necessarily say anything about your level of intelligence so much as how your mind works, so do not take it as a negative sign if your line wavers too much.
If your head line has a slight upward arc to it rather than the traditional downward arc, this can be a way of letting you know that your intelligence level is increasing, that you possess great mental powers, or that you soon will if you are reading it as a time line for the future. Another thing which can symbolize great mental prowess is another head line running consecutively parallel to the original. This line will usually be much smaller, but if it meets the same pattern as the head line, it is a way of showing you that essentially, all of your intelligence was not enough to fit on one line.
The head line does not always just speak of intelligence though, but also sometimes of your predisposition to learning in general. A good indication of this would be if your life line intersected with your head line. This would indicate a life that is filled with the desire to continue to pursue knowledge.