not nd filter
GND
graduated
neutral density filter
would not advise you to get screw in one
best to get cokin from cathay photo (check ortega's thread in "others" section for pricing, roughly)
or tianya from
ppcp, here
those are slide in filters with holders
much more versatile
for screw in gnd your horizon/graduation is fixed, cannot adjust as and when you like, quite du lan to use
if your budget is higher than mine, seeing how i am a poor student, you may consider more expensive ranges like lee or singh-ray, i think, but not sure where to get those