Comparing and Merging Files
2.4.1 Controlling Side by Side Format
The `-y' or `--side-by-side' option selects side by side
format. Because side by side output lines contain two input lines, the
output is wider than usual: normally 130 print columns, which can fit
onto a traditional printer line. You can set the width of the output
with the `-W columns' or `--width=columns'
option. The output is split into two halves of equal width, separated by a
small gutter to mark differences; the right half is aligned to a tab
stop so that tabs line up. Input lines that are too long to fit in half
of an output line are truncated for output.
The `--left-column' option prints only the left column of two
common lines. The `--suppress-common-lines' option suppresses
common lines entirely.