Where I can find a Major Key’s relative Minor Key?

Piano Companion has interactive Circle of Fifths which provides lots of useful information related to Major and Minor scales and especially how they relate to each other.

Open up Circle of Fifths and take a look at outer circle and inner circle.

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The outer circle lists Major scales, e.g. C Major (#1) and G Major (#3) scale are major scales. The C Major has related scales: F Major and G Major.

The Inner circle contains Minor scale keys and displays how it’s related to Major scale, e.g. A Minor (#2) is related to C Major (#1) or E Minor (#4) is related to G Major (#3).