parallel lines

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If a vector cross product zero , the vectors are parallel right?
But what's a good value for messuring that the product is zero?

If (cx>0.00001) And (cy>0.00001) And (cz>0.00001) ?
Short and long vectors will not have the same 'parallellity tolerance' :)

Should I calculate the magnitude first and do something like:

If (cx>0.00001*mag) And (cy>0.00001*mag) And (cz>0.00001*mag) ?

But the magnitude would be close to or zero also, so I'd have to check that against another threshold first. What threshold?