Read the signed numbers first
A hypothetical Sun at 18.4° north and Jupiter at 18.0° north are parallel by 0.4°. If both were south at those magnitudes, the same rule would apply. Change one to south and the relationship becomes a contra-parallel. Declination is latitude on the equatorial sky, not the latitude of a planet above the zodiac’s ecliptic plane.
The orb here is the absolute difference between two signed declinations, |δ₁ − δ₂|. Both must have the same sign. For a 1° orb, 18.4° N and 17.4° N sits on the boundary. A pair at 18.4° N and 18.0° S cannot be called parallel under this convention even though their magnitudes are close.
Why a parallel can hide from a zodiac wheel
A conventional chart wheel places planets around the ecliptic by longitude. Equal north–south distance from the celestial equator is a separate condition and cannot be read reliably from how close the glyphs look around that ring. Ecliptic latitude matters too. Computing declination from longitude alone as though every planet sat exactly on the ecliptic can misclassify a pair, especially one containing the Moon.
Interpretation and limits
Astrologers often read a parallel as a conjunction-like connection. That analogy is an interpretive rule, not a claim of literal conjunction or causal influence. The pair could be far apart in right ascension and in the visible sky. Read its zodiac aspects separately, and keep the selected orb in view. This page uses planetary body centres and excludes house cusps, Ascendant, and Midheaven; adding those points would require an exact place and time.