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Rapport occurs when two or more people feel that they are in sync or on the same wavelength because they feel similar or relate well to each other. Rapport is theorized to include three behavioral components: mutual attention, mutual positivity, and coordination.
The word stems from the old French verb rapporter which means literally to carry something back; and, in the sense of how people relate to each other means that what one person sends out the other sends back. For example, they may realize that they share similar values, beliefs, knowledge, or behaviors around politics, music or sports.
There are a number of techniques that are supposed to be beneficial in building rapport such as: matching your body language (i.e., posture, gesture, etc.); maintaining eye contact; and matching breathing rhythm.
A classic if unusual example of rapport can be found in the book Uncommon Therapy by Jay Haley, about the psychotherapeutic intervention techniques of Milton Erickson. Erickson developed the ability to enter the world view of his patients and, from that vantage point (having established rapport), he was able to make extremely effective interventions (to help his patients overcome life problems).
Building rapport is one of the most fundamental sales techniques. In sales, rapport is used to build relationships with others quickly and to gain their trust and confidence. It is a very powerful tool that veteran salespeople naturally employ, which allows them to close more deals with less effort.
Mirroring means getting into rhythm with the person on as many levels as possible.
Emotional Mirroring - Empathizing with someone's emotional state by being on 'their side'. You must apply the skill of being a good listener in this situation so as you can listen for key words and problems that arise when speaking with the person. This is so you can talk about these issues and question them to better your understanding of what they are saying and show your empathy towards them (Arnold, E and Boggs, josh. 2007).
Posture mirroring - Matching the tone of a person's body language not through direct imitation, as this can appear as mockery, but through mirroring the general message of their posture and energy.
Tone and Tempo Mirroring - Matching the tone, tempo, inflection, and volume of a person's voice.
Giving gifts or doing favors without directly asking for something in return can trigger feelings of obligation 
Commonality is the technique of deliberately finding something in common with a person or a customer in order to build a sense of camaraderie and trust. This is done through shared interests, dislikes, and situations. 
- Stewart, Dorothy (1998). Gower handbook of management skills. Gower Publishing. p. 282. ISBN 0-566-07889-9.
- Tickle-Degnen, Linda; Rosenthal, Robert (1990). "The nature of rapport and its nonverbal correlates". Psychological Inquiry 1: 285–293.
- Manser, Martin; Turton, Nigel (1998). Advanced Learners Dictionary. Wordsworth Editions. p. 574. ISBN 1-85326-763-5.
- "Rapport - Definition". Merriam Webster. Retrieved 22-Mar-2011.
- Graham, Colly. "Building Rapport". Retrieved 2010-09-22.
- "The Law of Reciprocity".
- DeGroot, Bob. "Establish trust and rapport". Retrieved 2010-09-22.