Mercury also has a special role in how a chart comes to be. In the account Human Design gives of imprinting, Mercury programs the Personality Crystal from the moment it enters the body until birth, which is why this planet is tied so closely to the expansion of consciousness and to thinking itself. Where Mercury sits, something in you is trying to get said.
On the Personality Side
Your Personality Mercury is the theme you consciously need to communicate in this life. It is what you find yourself talking about, teaching, writing, arguing for and returning to in conversation, and the gate tells you the subject. People with Mercury in a Throat gate often feel this as a plain need to speak; people with it in an awareness center feel it as a need to think something through out loud.
On the Design Side
Your Design Mercury also has something to say, but it says it without your knowledge. These are the things you find yourself telling people without quite knowing why, the message your body delivers through your presence, your tone and your habits of speech. Others often receive this communication more clearly than you send it.
Reading It In Your Chart
Find the Mercury glyph on each side of the chart and note the gate and line. Read the gate as a subject rather than a trait: this is what you are here to communicate, and the line tells you how. Then look at which center the gate belongs to. A Mercury in the Ajna is a thinker who needs to explain; in the Solar Plexus it communicates feeling; in the G Center it communicates identity and direction. In every case, the not-self version is talking about it to fill silence, and the correct version is saying it when Strategy and Authority say the moment is right.
Mercury Among the Others
Human Design groups Mercury with Mars and Jupiter as the planets of outer authority, the part of you that has something to offer others once your own inner authority is settled. Mercury is the voice of that offering.