Planets are parallel when they are have the same Declination, and both are on the same hemisphere, north or south of the equator. They could be anywhere in the zodiac, with or without a longitudinal aspect, but will be parallel when they are the same number of degrees from the equator and on the said side of it, north or south.
Since planets that are in conjunction are also often parallel, this declination aspect is often interpreted as if the planets were conjunct, no matter where they are in the zodiac. However, the conjunction aspect is based on having the same sign in common, which parellel planets often don't. Another way to look at it is simply that they are sharing a resonance or energy; there is a connection between them that may or may not be seen in the chart's zodiacal aspects.
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