In astrology the planet Mercury is the messenger, the communicator; the planet who links all the others together. He symbolises the way we think and learn, and says something about our communication patterns. Mercury is a planet of speed; he is often happy in quick signs that allow him to make lightning connections and enjoy the thrill of new knowledge at speed. He also however has a deeper, darker, side. Mercury, as the god Hermes, was the only god able to travel to the underworld in Greek mythology. This is symbolic of the power of our mind; it is through our thoughts and understanding that we can travel to the deepest recesses of our subconscious and discover what lies hidden.
Mercury in Aries for example shows a quick, assertive mind, one that rushes into new concepts and may enjoy a lively debate. Mental competition may be enjoyed, and this person will not shy from blurting out their thoughts, particularly if they feel strongly on the topic. Mercury in Capricorn in contrast will give a slower, more careful turn of thought. This placement has a reputation for facility in mathematics and this may well be related to its step by step thought processes, as it gradually builds up knowledge in a logical and organised fashion.
With Mercury in Pisces, learning may come almost by osmosis, as the mind soaks up what is around it. This Mercury placement may lack boundaries in its thoughts, and easily dive in and out of many different areas of knowledge. I have known many fine linguists with this placement; there is something in the versatility and duality of this placement that allows a language to be soaked up. For Mercury in Pisces immersion in a topic of interest may be the best form of learning.
Mercury in Leo might be a very creative thinker, someone who takes great pride in their opinions and enjoys espousing them in dramatic fashion. When Mercury is in the fixed signs, some stubbornness of thought can be present, and it might be difficult to change one's mind, once a course has been set.
Whatever sign your Mercury is placed in, his position in your horoscope will give you an indication of the area of life in which he expresses most naturally. In the 6th house for example, Mercury will play a powerful role in your working life and daily routines, which are likely full of learning and communications! Mercury in the 9th might suggest that the study of foreign cultures or philosophical systems would be highly appropriate, as your mind has a strong need to go beyond its known experience.