In displays that use Trinitron or Diamondtron technology, pixels are arranged in vertical stripes. These designs rely on the precision of the electron gun for horizontal separation, so the mask (called an aperture grille in these displays) only has to keep the vertical stripes separate.
The "lines" are shadows cast by thin damper wires that keep all the vertical aperture grille "strips" from moving around.
Toejam31
<font color=red>First Rig:</font color=red> <A HREF="http://www.anandtech.com/mysystemrig.html?rigid=17935" target="_new"><font color=green>Toejam31's Devastating Dalek Destroyer</font color=green></A>
<font color=red>Second Rig:</font color=red> <A HREF="http://www.anandtech.com/mysystemrig.html?rigid=15942" target="_new"><font color=green>Toey's Dynamite DDR Duron</font color=green></A>
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<font color=purple>"Some push the envelope. Some just lick it. And some can't find the flap."</font color=purple>