I've had a Thinkpad T61 14" wide screen laptop for about 4 years and I really liked it - the size of the computer was about ideal for me - small enough that I can use it on an airplane, but a big enough screen that I don't feel too cramped.
The T61 was starting to age, so I figured it seemed like a good time to replace it with a new thinkpad. I ordered a T420, which arrived today.
I was really surprised to see that while the machine itself seems to be of similar dimensions, the screen seems to be a good bit smaller: About the same width, but between 1/2 and 3/4 inch less height. I'd assumed that this laptop would have the same size screen as my old one.
Is this a design change from the older systems?
The aspect ratio of the LCD screens has transitioned from 4:3 -> 16:10 -> 16:9, so there is a dimension change from the T61 to the T420. The T61/T400/T410 models used 16:10 screen and the T420 moved to the 16:9 LCD format.
The 4:3 format had been a standard of the industry for many generations, the growth of the LCD TV market has largely precipitated the shift from 16:10 and then 16:9 ratio standard. (Also 4:3 aspect ratio were used for many years due to the inherent design of the CRT, which had the vaccum tube which due to the vaccum was better suited to have a square shaped tube. This size ratio of the CRT screen was kept when monitor moved to the LCD technology, much like the VGA port (which is also designed for the CRT analogue signal)).
T520 which has the 15.6 inch 16:9 aspect ratio laptop is very similar to the 15.4 inch widescreen T60/T61/T500's, but it also relinquishes some height due to the dimension change from 16:10 to the 16:9 standard.
If you want something lighter than the T61 get the T420s, which is a slim model and weighs only 2 kg or there about.