Print every 2 lines with sed

sed  'n;n;N;d' /etc/passwd

n;n; command prints 2 lines and the 3rd line is present in the pattern space.
N command reads the next line and joins with the current line.
d deletes the entire stuff present in the pattern space.

With this, the 3rd and 4th lines present in the pattern space got deleted. Since this repeats till the end of the file, it ends up in printing every 2 lines.