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It is easy to leave unnecessary spaces at the end of a line without realizing it. In most cases, this trailing whitespace has no effect, but there are special circumstances where it matters.
You can make trailing whitespace visible on the screen by setting the
Emacs displays trailing whitespace in the face
Trailing whitespace is defined as spaces or tabs at the end of a line. But trailing whitespace is not displayed specially if point is at the end of the line containing the whitespace. (Doing that looks ugly while you are typing in new text, and the location of point is enough in that case to show you that the spaces are present.)
To delete all trailing whitespace within the current buffer's restriction (see section AC.22 Narrowing), type M-x delete-trailing-whitespace RET. (This command does not remove the form-feed characters.)
Emacs can indicate empty lines at the end of the buffer with a
special bitmap on the left fringe of the window. To enable this
feature, set the buffer-local variable
nil value. The default value of this variable is
controlled by the variable
by setting that variable, you can enable or disable this feature
for all new buffers.