Ground copper pour for Crystals and Real time clocks
I generally try to keep ground pours away from crystal traces, especially if they're more than a few cm long. One reason is to keep stray capacitances to a minimum &mdash; even a few pF can be significant. Another reason is to avoid coupling currents that might be flowing in the ground plane into the oscillator, which could increase clock jitter (which may or may not be significant in a particular application).