Gemini is ruled by Mercury. Common associations are the colour yellow, and the number 5. It rules the hands, arms and lungs in the physical body, its zodiac symbol is the twins. The sign is connected to communication and information. Its keywords are I think.  

Geminis are often the ones who remind you that life doesn’t have to be serious and heavy. Their ability to gather information and communicate can be nothing short of outstanding if the planet is well aspected in the chart. There is no bigger crime for a Gemini than to deny and repress its natural curiosity, and for those who don’t get their questions answered as a child, it can cause long-term damage to their intellectual  confidence. Ideally, they should never stop learning and always be doing one course or another, whether it’s nuclear physics or basket weaving. It makes no difference what they're interested as long as it’s keeping the curiosity alive. No one taught me more about the best of this sign than my youngest daughter who was fully conversational before she had even turned two years of age. Now in myth, the trickster Loki is associated with Gemini and with good reason, even the youngest of them can set up situations with precision, their mental cogs will be turning and then they'll ask a question…frequently a leading one and then a situation will play out with her usually getting the mental better of someone. They tell and get jokes from a yound age and will be verbally on par with many adults. 

Gemini is represented by the twins which indicates there are two sides to this sign yet the thing about twins is they carry the same genetics and the two sides of Gemini are not that different in nature. Some may say they are split into good and bad, but the Gemini will only do this if it hasn't been safe to express himself fully, tell them to shut up once to often and they might just do that and then we will have a classic case of Jekyll and Hyde. 

Gemini is happiest when communicating. They're fun, childlike, and have a light carefree attitude towards life. They will play games and tease you; if anyone is going to instigate a performance from Leo or a fight from Aries, it’s a Gemini. They rarely think about consequences because they are far too busy thinking about everything else. The two most common phrases I ever heard come out of the mouth of a Gemini Sun are “I didn’t mean to say it like that” and “I didn’t think”, which is a laugh in itself because all they ever do is think. Don’t be fooled by the apparent innocence, it’s all a game; but if they do get the better of you, don’t take it to heart because this sign is rarely malicious in nature. They are mischievous without a doubt and playful til their last breath, they can be silly and childish, flirty and fickle but they are supposed to be. It's part of the Gemini quest to stay eternally youthful. To take the inner, amazed child from Gemini is to take away the very essence of who they are.

Like the other signs, we all have Gemini somewhere in the natal chart and by seeing which house it falls into we can get our clues about where we are most likely to be curious. Should we be lucky enough to get a planet in Gemini, we need to check aspects to that planet to see if it's supported or suppressed. Mercury in Gemini with good aspects to Uranus can indicate pure genius; Saturn will make Gemini insecure about their knowledge base, and Mars will like nothing better than to provoke a good intellectual debate.