“Six-pack” refers to the 6 basic instruments in light plane. To describe them, pilot in aviation usually use an imaginary “T”, also known as “basic-T“ to explain the relative position of them.
Generally speaking,  Attitude Indicator (AI) is on the top and in the middle of the “T”. On its left is the  Airspeed Indicator (ASI) and on its right is the  Altimeter (ALT). In the bottom left corner, that is below the Airspeed Indicator is the  Turn Indicator (TI); In the bottom right corner, that is below the Altimeter is the  Vertical Speed Indicator (VSI); Lastly, below the Attitude Indicator is the  Heading Indicator (HI) [aka directional indicator (DI)]
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