This (sometimes controversial) issue pops up every so often.

From any general line (one-faced, two-faced, 3&1, or a WAVE), Partners are generally defined as the Ends and Adjacent Centers. This holds true for calls such as PARTNER HINGE, PARTNER TRADE, and PARTNER TAG.

This is the "Formation"-relative Partner, who may or may not be your original Partner, and could even be dancing the same Role (i.e., Boy or Girl).

For the purposes of call such as RIGHT AND LEFT GRAND, and LEFT ALLEMANDE, you need to look at the 8-dancer formation as a "Circle", and the Boy's partner is the lady counter-clockwise around the circular formation.

And, of course, not all callers may subscribe to this, nor understand it. So, bottom line, do what your caller wants. :-)