"Sign (mathematics)" is a title based upon conjecture.
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Sign was the property of being positive, negative, or zero.
E-Space had the quality of being a "negative universe" in contrast to N-Space's positive coordinates. (TV: Full Circle), Warriors' Gate, PROSE: Castrovalva) The Gateway had a zero coordinate value, (TV: Warriors' Gate) as did the planet Time. (TV: War of the Sontarans)