How to learn to tune my guitar by ear?

Question by TheonethingIhaveleft: How to learn to tune my guitar by ear?
I am wondering how I should start to learn how to tune my guitar by ear. I want to learn how to tune by ear because I don't always have my tuner with me and when I go to guitar stores the guitar there are not always tuned. I can tune an already tuned guitar to drop D

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