Here’s a good way to remember what the function is of a planet, sign, and house:
1. The PLANet, makes the PLAN.
2. The Sign, carries out that plan in the style of the sign. Think of your signature, or way you like to dress.
3. The House, is where and why the action occurs. It’s the vehicle. It contains the planet and sign.
The domicile planet for Mercury is Gemini, and co-rules Virgo.
I am using Mercury Aries in the 12th house as the example.
The Planet is Mercury. It’s plan for you is to think and communicate.
The Sign is Aries. You are going to carry out this plan by thinking fast, and furiously. By being innovative, brave, and courageous in expressing your thoughts.
The House is the 12th. Poetry may be written. Poetry may be chosen as the vehicle for expression because it quiets the loudness of Aries while still making a strong impact. Aries Mercury in the 12th, requires a less direct approach since its speaker is too forward and brusque for a verbal discourse, musing, or philosophical discussion for gentler minds.