Mercury tells what sort of mind you have, where your decisions are felt, and the type of decisions you make. It shows where your main interests are, where you voice your opinions, your thoughts. The type of writing you do, how you speak, how you listen, all come under Mercury's influence.
Mercury rules communications and transportation.

Mercury rules both Gemini and Virgo and will continue to do so until a new planet is discovered to take it's place in one of those signs (probably Virgo). Mercury therefore has all the characteristics of both these signs and their Solar houses, the 3rd and 6th, so that it rules both communications and services.

Mercury is mail, papers, agreements, books, bookkeeping, letters, the telephone, secretarial duties, mechanical objects such as the computer, adding machine and typewriters. Mercury operates as an agent for you.

With Mercury in Aries you write short, quick notes if you write at all, (Aries is usually too busy and can't find the time). In Cancer you write all the family news or perhaps political news. In Scorpio you may write all the lurid details of the most recent love affair or perhaps an intense letter on psychic experiences. A Sagittarian Mercury would more than likely write long, chatty letters.