isnt 1366 × 768 = wxga?
hd is usually 720,900,or 1080.
But which one for Dell's "High Definition LED Display with TrueLife"?
Dell markets a few resolutions under that heading, unfortunately, to confuse buyers. Most of their 15.6in screens come with either 1366x768 or 1600x900, depending on the buyer upgrades. Given the CPU in the Inspiron you gave the specs for, I'd probably say it was a 1366x768 model too.
This isn't specific to Dell, but in general, from what I've seen:
HD = 7xx lines of resolution
HD+ = 9xx lines of resolution (usually 1600x900)
FHD = 1xxx lines of resolution (usually 1900x1200 or 1900x1080)
Confusing as hell, if you ask me. Which is probably why they do it.
(my current laptop is 1900 x 1200, but is a 5yr old Pentium M 1.6ghz)
