I'm not sure about today's marking substance, but for many years, in the old days, it was nothing more than POWDERED CHALK, which I think is Calcium Carbonate. Today, some are using synthetic materials, or even spray paint.
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skill
Magnetic field lines don't cross.
Yes, usually. The lines are simply shown to illustrate direction and strength of the field.
A ruler
ther are two lines of symetry on a football pitch