Undo an unwanted change

`Ctrl` + `_`

All hotkeys for GNU Emacs

Files Read a file into Emacs	`Ctrl` + `X` then `Ctrl` + `F`
Files Save a file back to disk	`Ctrl` + `X` then `Ctrl` + `S`
Files Save all files	`Ctrl` + `X` then `S`
Files Insert contents of another file into this buffer	`Ctrl` + `X` then `I`
Files Replace this file with another file	`Ctrl` + `X` then `Ctrl` + `V`
Files Write buffer to specified file	`Ctrl` + `X` then `Ctrl` + `W`
Files Toggle read only status of buffer	`Ctrl` + `X` then `Ctrl` + `Q`
Leaving Emacs Suspend or iconify Emacs	`Ctrl` + `Z`
Leaving Emacs Exit Emacs completely	`Ctrl` + `X` then `Ctrl` + `C`
Multiple windows Delete all other windows	`Ctrl` + `X` then `1`
Multiple windows Scroll other window	`Ctrl` + `Meta` + `V`
Help Show commands matching a string	`Ctrl` + `H` then `A`
Help Describe the function a key runs	`Ctrl` + `H` then `K`
Help Describe a function	`Ctrl` + `H` then `F`
Help Get mode specific information	`Ctrl` + `H` then `M`
The minibuffer Abort command	`Ctrl` + `G`
Simple customization (then customize) Customize variables and faces	`Meta` + `X`
Error recovery Undo an unwanted change	`Ctrl` + `X` then `U`
Error recovery Undo an unwanted change	`Ctrl` + `_`
Error recovery Undo an unwanted change	`Ctrl` + `/`
Motion Scroll current line to center, top, bottom	`Ctrl` + `L`
Incremental search Search forward	`Ctrl` + `S`
Query replace Enter recursive edit	`Ctrl` + `R`
Incremental search Regular expression search	`Ctrl` + `Meta` + `S`
Incremental search Reverse regular expression search	`Ctrl` + `Meta` + `R`
The minibuffer Fetch previous minibuffer input	`Meta` + `P`
The minibuffer Fetch later minibuffer input or default	`Meta` + `N`
The minibuffer Complete and execute	`⤶ Enter`
Info Scroll reverse	`Del`
Motion Character - backward	`Ctrl` + `B`
Motion Character - forward	`Ctrl` + `F`
Motion Word - backward	`Meta` + `B`
Motion Word - forward	`Meta` + `F`
Motion Line - backward	`Ctrl` + `P`
Motion Line - forward	`Ctrl` + `N`
Motion Go to line beginning	`Ctrl` + `A`
Motion Go to line ending	`Ctrl` + `E`
Motion Sentence - backward	`Meta` + `A`
Motion Sentence - forward	`Meta` + `E`
Motion Paragraph - backward	`Meta` + `{`
Motion Paragraph - forward	`Meta` + `}`
Motion Page - backward	`Ctrl` + `X` then `[`
Motion Page - forward	`Ctrl` + `X` then `]`
Motion sexp - backward	`Ctrl` + `Meta` + `B`
Motion sexp - forward	`Ctrl` + `Meta` + `F`
Motion function - backward	`Ctrl` + `Meta` + `A`
Motion function - forward	`Ctrl` + `Meta` + `E`
Motion Go to beginning of buffer	`Meta` + `<`
Motion Go to end of buffer	`Meta` + `>`
Motion Scroll to next screen	`Ctrl` + `V`
Motion Scroll to previous screen	`Meta` + `V`
Motion Scroll left	`Ctrl` + `X` then `<`
Motion Scroll right	`Ctrl` + `X` then `>`
Motion Go to line	`Meta` + `G` then `G`
Motion Go to character	`Meta` + `G` then `C`
Motion Back to indentation	`Meta` + `M`
Killing and deleting Character (delete, not kill) - forward	`Ctrl` + `D`
Killing and deleting Word - backward	`Meta` + `Del`
Killing and deleting Word - forward	`Meta` + `D`
Killing and deleting Line (to end of) - backward	`Meta` + `0` then `Ctrl` + `K`
Killing and deleting Line (to end of) - forward	`Ctrl` + `K`
Killing and deleting Sentence - backward	`Ctrl` + `X` then `Del`
Killing and deleting Sentence - forward	`Meta` + `K`
Killing and deleting sexp - backward	`Meta` + `-` then `Ctrl` + `Meta` + `K`
Killing and deleting sexp - forward	`Ctrl` + `Meta` + `K`
Killing and deleting Kill region	`Ctrl` + `W`
Killing and deleting Copy region to kill ring	`Meta` + `W`
Killing and deleting then press char to kill through next occurence of char	`Meta` + `Z`
Killing and deleting Yank back last thing killed	`Ctrl` + `Y`
Killing and deleting Replace last yank with previous kill	`Meta` + `Y`
Marking Set mark here	`Ctrl` + `@`
Marking Set mark here	`Ctrl` + `Space`
Marking Exchange point and mark	`Ctrl` + `X` then `Ctrl` + `X`
Marking Set mark arg words away	`Meta` + `@`
Marking Mark paragraph	`Meta` + `H`
Marking Mark page	`Ctrl` + `X` then `Ctrl` + `P`
Marking Mark sexp	`Ctrl` + `Meta` + `@`
Marking Mark function	`Ctrl` + `Meta` + `H`
Marking Mark entire buffer	`Ctrl` + `X` then `H`
Query replace (then query-replace-regexp) Interactively replace a text string using regular expressions	`Meta` + `%` then `Meta` + `X`
Info Scroll forward	`Space`
Query replace Replace this one, go to next	`Y`
Info Go to next node	`N`
Query replace Replace all remaining matches	`!`
Query replace Back up to the previous match	`^`
Query replace Exit recursive edit	`Ctrl` + `Meta` + `C`
Multiple windows Delete all other windows (for frame)	`Ctrl` + `X` then `5` then `1`
Multiple windows Split window, above and below	`Ctrl` + `X` then `2`
Multiple windows Split window, above and below (for frame)	`Ctrl` + + `X` then `5` then `2`
Multiple windows Delete this window	`Ctrl` + `X` then `0`
Multiple windows Delete this window (for frame)	`Ctrl` + `X` then `5` then `0`
Multiple windows Split window, side by side	`Ctrl` + `X` then `3`
Multiple windows Switch cursor to another window	`Ctrl` + `X` then `O`
Multiple windows Switch cursor to another window (for frame)	`Ctrl` + `X` then `5` then `O`
Multiple windows Select buffer in other window	`Ctrl` + `X` then `4` then `B`
Multiple windows Select buffer in other window (for frame)	`Ctrl` + `X` then `5` then `B`
Multiple windows Display buffer in other window	`Ctrl` + `X` then `4` then `Ctrl` + `O`
Multiple windows Display buffer in other window (for frame)	`Ctrl` + `X` then `5` then `Ctrl` + `O`
Multiple windows Find file in other window	`Ctrl` + `X` then `4` then `F`
Multiple windows Find file in other window (for frame)	`Ctrl` + `X` then `5` then `F`
Multiple windows Find file read-only in other window	`Ctrl` + `X` then `4` then `R`
Multiple windows Find file read-only in other window (for frame)	`Ctrl` + `X` then `5` then `R`
Multiple windows Run Dired in other window	`Ctrl` + `X` then `4` then `D`
Multiple windows Run Dired in other window (for frame)	`Ctrl` + `X` then `5` then `D`
Multiple windows Find tag in other window	`Ctrl` + `X` then `4` then `.`
Multiple windows Find tag in other window (for frame)	`Ctrl` + `X` then `5` then `.`
Multiple windows Grow window taller	`Ctrl` + `X` then `^`
Multiple windows Shrink window smaller	`Ctrl` + `X` then `{`
Multiple windows Grow window wider	`Ctrl` + `X` then `}`
The minibuffer Complete as much as possible	`⭾ Tab`
Formatting Indent region (mode-dependent)	`Ctrl` + `Meta` + `\`
Formatting Indent sexp (mode-dependent)	`Ctrl` + `Meta` + `Q`
Formatting Indent region rigidly arg columns	`Ctrl` + `X` then `⭾ Tab`
Formatting Indent for comment	`Meta` + `;`
Formatting Insert newline after point	`Ctrl` + `O`
Formatting Move rest of line vertically down	`Ctrl` + `Meta` + `O`
Formatting Delete blank lines around point	`Ctrl` + `X` then `Ctrl` + `O`
Formatting Join line with previous (with arg, next)	`Meta` + `^`
Formatting Delete all white space around point	`Meta` + `\`
Formatting Put exactly one space at point	`Meta` + `Space`
Formatting Fill paragraph	`Meta` + `Q`
Formatting Set fill column to arg	`Ctrl` + `X` then `F`
Formatting Prefix each line starts with	`Ctrl` + `X` then `.`
Formatting Set face	`Meta` + `O`
Case change Uppercase word	`Meta` + `U`
Case change Lowercase word	`Meta` + `L`
Case change Capitalize word	`Meta` + `C`
Case change Uppercase region	`Ctrl` + `X` then `Ctrl` + `U`
Case change Lowercase region	`Ctrl` + `X` then `Ctrl` + `L`
The minibuffer Show possible completions	`?`
The minibuffer Regexp search backward through history	`Meta` + `R`
The minibuffer Regexp search forward through history	`Meta` + `S`
The minibuffer Edit and repeat the last command that used the minibuffer	`Ctrl` + `X` then `Esc` then `Esc`
The minibuffer Activate menu bar items in text terminals	`F10`
Buffers Select another buffer	`Ctrl` + `X` then `B`
Buffers List all buffers	`Ctrl` + `X` then `Ctrl` + `B`
Buffers Kill a buffer	`Ctrl` + `X` then `K`
Transposing Transpose characters	`Ctrl` + `T`
Transposing Transpose words	`Meta` + `T`
Transposing Transpose lines	`Ctrl` + `X` then `Ctrl` + `T`
Transposing Transpose sexps	`Ctrl` + `Meta` + `T`
Spelling check Check spelling of current word	`Meta` + `$`
Tags Find a tag (a definition)	`Meta` + `.`
Tags Find next occurence of a tag	`Ctrl` + `U` then `Meta` + `.`
Tags Continue last tags search on query-replace	`Meta` + `,`
Shells Execute a shell command	`Meta` + `!`
Shells Execute a shell command asynchronously	`Meta` + `&`
Shells Run a shell command on the region	`Meta` + `\|`
Rectangles Copy rectangle to register	`Ctrl` + `X` then `R` then `R`
Rectangles Kill rectangle	`Ctrl` + `X` then `R` then `K`
Rectangles Yank rectangle	`Ctrl` + `X` then `R` then `Y`
Rectangles Open rectangle, shifting text right	`Ctrl` + `X` then `R` then `O`
Rectangles Black out rectangle	`Ctrl` + `X` then `R` then `C`
Rectangles Prefix each line with string	`Ctrl` + `X` then `R` then `T`
Abbrevs Add global abbrev	`Ctrl` + `X` then `A` then `G`
Abbrevs Add mode-local abbrev	`Ctrl` + `X` then `A` then `L`
Abbrevs Add global expansion for this abbrev	`Ctrl` + `X` then `A` then `I` then `G`
Abbrevs Add mode-local expansion for this abbrev	`Ctrl` + `X` then `A` then `I` then `L`
Abbrevs Explicitly expand abbrev	`Ctrl` + `X` then `A` then `E`
Abbrevs Expand previous word dynamically	`Meta` + `/`
Miscellaneous Numeric argument	`Ctrl` + `U` then `Num 0` - `Num 9`
Miscellaneous Negative argument	`Meta` + `-`
Miscellaneous then char: Quoted insert	`Ctrl` + `Q`
International character sets Enable or disable input method	`Ctrl` + `\`
Info Enter the Info documentation reader	`Ctrl` + `H` then `I`
Info Find specified function or variable in Info	`Ctrl` + `H` then `S`
Info Beginning of node	`B`
Info Go to previous node	`P`
Info Move up	`U`
Info Select menu item by name	`M`
Info Select menu item by number	`Num 1` - `Num 9`
Info Follow cross reference (return with 1)	`F`
Info Return to last node you saw	`L`
Info Return to directory node	`D`
Info Go to top node of Info file	`T`
Info Go to any node by name	`G`
Info Run info tutorial	`H`
Info Look up a subject in the indices	`I`
Info Search nodes for regexp	`S`
Info Quit Info	`Q`
Registers Save region in register	`Ctrl` + `X` then `R` then `S`
Registers Insert register contents into buffer	`Ctrl` + `X` then `R` then `I`
Registers Save value of point in register	`Ctrl` + `X` then `R` then `Space`
Registers Jump to point saved in register	`Ctrl` + `X` then `R` then `J`
Keyboard macros Start defining a keyboard macro	`Ctrl` + `X` then `(`
Keyboard macros End keyboard macro definition	`Ctrl` + `X` then `)`
Keyboard macros Eappend to last keyboard macro	`Ctrl` + `X` then `E`
Dealing with Emacs Lisp Eval sexp before point	`Ctrl` + `X` then `Ctrl` + `E`
Dealing with Emacs Lisp Eval current defun	`Ctrl` + `Meta` + `X`
Dealing with Emacs Lisp Read and eval minibuffer	`Meta` + `:`