In addition, if Emacs is currently inside a recursive editing level, square brackets ([...]) appear around the parentheses that surround the modes. If Emacs is in one recursive editing level within another, double square brackets appear, and so on. Since recursive editing levels affect Emacs globally, not just one buffer, the square brackets appear in every window's mode line or not in any of them. See Recursive Edit.
Non-windowing terminals can only show a single Emacs frame at a time (see Frames). On such terminals, the mode line displays the name of the selected frame, after ch. The initial frame's name is F1.
cs states the coding system used for the file you are editing. A dash indicates the default state of affairs: no code conversion, except for end-of-line translation if the file contents call for that. = means no conversion whatsoever. Nontrivial code conversions are represented by various letters--for example, 1 refers to ISO Latin-1. See Coding Systems, for more information. If you are using an input method, a string of the form i> is added to the beginning of cs; i identifies the input method. (Some input methods show + or @ instead of >.) See Input Methods.
When you are using a character-only terminal (not a window system), cs uses three characters to describe, respectively, the coding system for keyboard input, the coding system for terminal output, and the coding system used for the file you are editing.
When multibyte characters are not enabled, cs does not appear at all. See Enabling Multibyte.
The colon after cs can change to another string in certain circumstances. Emacs uses newline characters to separate lines in the buffer. Some files use different conventions for separating lines: either carriage-return linefeed (the MS-DOS convention) or just carriage-return (the Macintosh convention). If the buffer's file uses carriage-return linefeed, the colon changes to either a backslash (\) or (DOS), depending on the operating system. If the file uses just carriage-return, the colon indicator changes to either a forward slash (/) or (Mac). On some systems, Emacs displays (Unix) instead of the colon even for files that use newline to separate lines.
You can customize the mode line display for each of the end-of-line formats by setting each of the variables eol-mnemonic-unix, eol-mnemonic-dos, eol-mnemonic-mac, and eol-mnemonic-undecided to any string you
