Ctrl-Up/Down: Zoom board in/out 1%.

Ctrl-Shift-Up/Down: Zoom board in/out 5%.

Shift-A: Toggle alert when then puzzle is solved.

T: Toggle display of the timer. Click the timer to pause/resume.

Shift-P: Toggle display of the position in the history (for identifying positions in the puzzle). Right-click the Toggle Bookmark button for the same command.

H: Scroll the 'highlight' setting. Highlight user moves, solver moves, or neither. Highlight info is included in the history.

G: Toggle grid display. (The grid display is modelled on Simon Tatham's.)

Ctrl-Shift-A: Toggle animation of x's in the grid display. X's drawn by undo or redo will be animated. All x's drawn by go to previous or next bookmark, or by the solver in some cases, will be animated if only x's are drawn.

D: Toggle alternate normal display. Right-click the Toggle Grid button for the same command.

Shift-F: Scroll the font. One of the fonts is unavailable with Internet Explorer.

Ctrl-Shift-F: Scroll the darkening value of the current font. Values are 0, 10%, 20% ... 100%. There is a separate setting for each of the three fonts.

Shift-B: Scroll the container background color.

A: Toggle shading. A shade is applied to a square and indicates whether the square is thought to be inside or outside the loop that is the solution to the puzzle. Shades can be used to deduce lines and x's, and other shades (see previous link). They are not included in the history or in locks, and are not considered by the solver or error checker; they will be included in an exported SVG file. Ctrl-A: If there are calculated shades on the board, clear them; otherwise (turn shading on and) calculate the shades for the current position. Right-click the Toggle Shading button for the same command. The shades are calculated based only on existing lines and x's and trivial x's deduced by the solver. The only rule used is two adjacent squares with a line/x between them must have opposite/equal shades. This implies that no deduction can be made unless the shade of one of the two squares is known. At the start of the calculation, the squares with known shades are those shaded by the user, and the two rows and columns of virtual squares beyond the four sides of the board; the latter are outside the loop (there is a setting that determines whether these shades are visible). No error checking is done. If a shade is produced that violates the one rule being used, it implies that there is an error on the board (an incorrect line, x, or shade).

Left/Right, Ctrl-Z/Y: Undo/Redo. Shift-Left/Right: Go to start/end of user or solver set of moves. Right-click the Undo/Redo buttons for the same commands.

Up/Down: Go to previous/next bookmark. The beginning and end of the history function as bookmarks. If Up is pressed and all lines and x's disappear, that's why, just press Down to recover if it wasn't intended. Shift-Up/Down: Go to beginning/end of the history. Right-click the Previous/Next Bookmark buttons for the same commands.

Mousewheel Up/Down: Depending on the Mousewheel setting, either undo/redo or go to previous/next bookmark. If the setting is 'Default', if auto-solve is off, undo/redo, else go to previous/next bookmark. With either button pressed, that behavior is reversed (move the mouse pointer away from the board before pressing the button; this may not work in Safari or IE).

Enter: Add/Remove bookmark at the current position in the history. Ctrl-Enter: Truncate the history at the current position. Ctrl-click the Toggle Bookmark button for the same command.

Y: Play back the history from the current position; if at the end of the history, play back from the start. Ctrl-click the Redo button for the same command. Press any key, or click anywhere, to stop the playback. There are settings for the duration of the playback and the interval between moves; if the former is non-zero, the latter is calculated. Another setting determines whether there is an alert if the puzzle is solved when playback is completed.

V: Execute the solver. Ctrl-V: Execute the solver at full strength (all relevant settings considered on except those with a value of 'Never') for one move. Right-click the Solve button for the same command. Ctrl-Right-click the Solve button to execute the solver at full strength for as many moves as possible (the puzzle will be completely solved if possible). Shift-Right-click the Solve button to execute the solver at absolute full strength (all settings considered on regardless of their values, except for 'Assume Only One Solution').

Shift-V: Scroll the solver mode. 1 = solve one move when V is pressed, 2 = solve as much as possible when V is pressed, 3 = solve as much as possible automatically whenever the board is changed by a mouse click. 1 is the default.

Ctrl-Shift-V: Scroll the solver mode for locked positions.

Shift-E: Toggle extended logic (off by default). When the 'Use Patterns' setting is on, the 'Extended Logic' setting is ignored and extended logic is used.

Shift-L: Scroll the solving of loops - placing x's in places where a line would complete or cause a loop that is not the solution (on by default).

Shift-R: Scroll the solving of rows and columns - making deductions based on the fact that the solution must pass through each row and column an even number of times (*) (off by default). See 'More about the solver', below, for info.

Shift-N: Scroll the use of patterns (off by default). See 'More about the solver' for info.

Shift-O: Scroll solving based on the puzzle having only one solution (*) (off by default). See Settings and 'More about the solver' for info.

Ctrl-O: Toggle the 'Assume Only One Solution' setting (on by default). See Settings and 'More about the solver' for info.

Shift-X: Scroll the solver's "add x's" mode. 0 = no x's are added to the board; 1 = only non-trivial x's are added; 2 = all x's are added; 3 = only trivial x's - and no lines - are added. 2 is the default. 1 isn't working perfectly, trivial x's are sometimes being added. If the solver mode is One Move, this setting is ignored and only non-trivial x's are added. 3 is mainly intended for use with auto-solve and the grid display.

Ctrl-B: Toggle adding a bookmark before every solver move.

1: Lock the current position.

2: Reset to the last locked position; if at the last locked position, unlock. A position can be locked at any point in the history, and if it's unlocked by a reset, the history will be unchanged.

3: Unlock all locked positions. After the positions are unlocked, the solver will execute if auto-solve is on.

4: Reset to the last locked position and switch the first move. Right-click the Reset button for the same command. If auto-solve is on, the solver will execute immediately after the switch. Because this command might inadvertently result in the loss of moves, it must be enabled with a setting.

Shift-4: Enable/disable Reset And Switch (off by default).

K: Produce a link to the current puzzle in its current state. If 'This tab' is clicked, the url of the current page will be updated in the browser's address bar (this will probably not be noticeable); reload the page to return to that state. The complete history is included in the url, so if the history is very lengthy the url may be too long to process in some browsers, though this has not been observed in Chrome or Firefox. Click either of the text fields to copy its contents to the clipboard.

S: Save current puzzle to local storage in its current state.

Right-click the 'Save' button, or choose 'Browse saved puzzles' from the 'More Commands' menu, to load or remove saved puzzles. Click 'Export' for a set of links to all, or selected, saved puzzles including histories. Click 'Import' to import puzzles that were previously exported, or to import puzzles from any block of text that contains puzzle codes. Click the puzzle count to display row numbers. Click the plus sign, then 'Show user-defined codes,' or right-click the plus sign, to open a column of editable cells to the left of the puzzle codes column. The meaning of these codes is defined by the user. Whitespace is not allowed in the codes. A query consisting of a list of (comma) delimited codes, or a regular expression, can be created that, when 'active' (*), can be used by the 'Export' and 'Clear' commands, and when scrolling through saved puzzles (by right-clicking the Load button). The format of the query is an optional string of one or more single-character flags followed by a space, followed by the list of codes or regular expression. The default delimiter for a list of codes is a comma. Any non-whitespace, non-alphanumeric character, except a minus sign, can be used; include the character in the flags. By default, the query selects all puzzles with codes that are in the list or that match the regular expression; include a minus sign in the flags to reverse that. By default, the query is case-sensitive; include 'i' in the flags to make it case-insensitive. Include 's' in the flags to assign special meanings to these characters in a list of codes: #) any digit; @) any letter; ?) any character; *) any number of characters including none. To use any of these characters as a literal with the 's' flag, precede it with a backslash. Include 'r' in the flags to indicate a regular expression. Whitespace is not allowed in a list of codes; it's stripped from a regular expression. To match puzzles that have no user-defined code, begin or end the list of codes with the delimiter. (*) The query will be considered 'active' only when it's enabled and visible when the 'Browse Saved Puzzles' window is displayed.

Shift-S: Remove current puzzle from storage (if previously saved).

L: Load current puzzle (if previously saved). When a puzzle is drawn it will be loaded automatically if previously saved, unless the url contains a history. A puzzle is identified by its 'puzzle code', which is always included in the url (see Notes, below). Right-click the Load button to load the first puzzle below the current one in the list of saved puzzles sorted from last (most recently) saved to first saved. If the current puzzle hasn't been saved, the first puzzle in the list is loaded. If there is an active query (see 'Browse Saved Puzzles', above), only selected puzzles will be in the list, unless the query selects no puzzles. If the button flashes yellow, it means that the current puzzle is saved and is, or is below, the last puzzle in the list. Shift-Right-click the Load button to load the first puzzle in the list. (This may not work in Firefox.)

0: Clear the board. The history is also cleared.

Shift-0: Show the solution if it's known. The history and locked positions will be discarded, so save the puzzle first if that's not wanted.

9: Load the SAT Slitherlink Solver. Depending on the browser, the solver might open in a new window, or maybe not at all. Use the 'More Commands' menu to reliably open the SAT solver in a new tab.

Spacebar: Search the puzzle for errors. If there is only one solution and the solution is known, the puzzle background will flash green or red, otherwise it will flash yellow or red. With Ctrl-Spacebar (or Right-click the Check button), the first search is for errors that can be determined without knowing the solution; the first such error found will blink. Then, if there is only one solution and the solution is known, the first error added to the board will blink and remain highlighted. The main reason for the first search is that it's sometimes possible to draw a single loop that passes through the indicated number of sides of every numbered square – except one. And on large boards, that one square can be hard to find. (Here are two examples.) Whether or not the solution is known is indicated by the Check button. If the solution isn't known, the checkmark icon will be noticeably smaller than the other icons.

UPDATE: The striped checkmark icon indicates that the primary solver couldn't solve the puzzle, and the puzzle loaded with a history that contained the solution. Since the SAT solver solves all puzzles, the small clear icon indicates that the puzzle has no solution or that the SAT solver failed to excecute. If the small icon is filled with light gray, it means the SAT solver is executing.

Ctrl-E: Toggle the 'Check for errors after user input' setting. If it's on (which is not the default), and no position is locked, the puzzle will be searched for errors automatically whenever the board is changed by a mouse click or drag. If one or more positions are locked, the solver will check for errors when all positions are unlocked. If an error is found, it does not necessarily imply that the last user move was an error; for example, the puzzle might have loaded with a history containing an error. Right-click the 'Check' button to see the first error added to the board.

C: Toggle display of connected lines. When on, hover over a line to show all connected lines. This feature can be useful on large puzzles.

P: (Right-click 'New' for the same command. See immediately below for more on the 'New' button.) Prompt for a puzzle code, and draw or solve the puzzle. See the Notes section for the definition of 'puzzle code'. Unless the puzzle is very small, the fastest way to manually enter a puzzle code will be to 1) omit the width and height from the textarea, and 2) enter one row of characters for each puzzle row, using one character for each square, with a minus sign or period for each blank square (e.g., '3---2' instead of '3c2'). If this method is used, 1) only these characters are permitted: "-._a0123", and line feed (carriage return is ignored); 2) if the width is not specified above the textarea, it will be set to the length of the first line; 3) when a character is appended to the last line so that the length of the line becomes equal to the width, a line feed will be appended automatically; 4) if a valid key is held down (after the width is known), the character will repeat only to the end of the line, making it easy to fill the remainder of a line with blanks; 5) the lines will be adjusted when characters are inserted or deleted anywhere in the puzzle code; 6) if the width and height are specified above the textarea, the puzzle code can be entered and submitted using only these keys: "-._a0123" (without pressing Enter or clicking Draw). If the width and height are specified in the textarea, all whitespace is allowed and ignored. If 'Submit on Enter' is set to 'No', Shift-Enter submits the puzzle code. Right-click the 'Draw' button to display a solution immediately. The textarea is vertically resizable, but apparently not in IE 11.

If the 'Quick Paste Puzzle Code' setting is on, right-click 'New' and hold the mouse button down for an instant (> 200 ms) to paste a puzzle code and draw the puzzle. A puzzle code can also be dropped on the 'New' button.

No Key: A few settings, including 'Skip Puzzles', can only be changed in the Settings window. Some commands cannot be executed with a keystroke.

Export the puzzle to an SVG file: The file will contain no javascript, so it can be used in a webpage like any other type of image file. The image will be a snapshot of the board in its current state. There are settings that determine whether the margin and border are preserved, by default they are not.

Restore last deleted puzzle: When a puzzle is deleted individually, it is saved as the last deleted puzzle. The puzzle will be restored with a new (last updated) timestamp. The 'Browse saved puzzles' window will be displayed, and the restored puzzle will be at the top of the list.

A 'puzzle code' is in the form: WxHdD:S, where W and H are integers indicating the width and height of the board (in squares, not points), D is a digit from 0 to 3 indicating the difficulty of the puzzle, and S is a string of characters that indicates where the numbers are placed on the board, from left to right and top to bottom. A lower case letter is used to indicate one or more blank squares: a = 1, b = 2, etc. Some sites require a letter for blank squares following the last number; here it's optional. The two characters indicating difficulty will be recongnized only at this site, and can be omitted.

For example, the code for the following puzzle is 6x4:e3b2e02233b1b.

In addition to Simon Tatham's site, a puzzle code can be entered at this site, which also features a very fine generator and solver. Currently, both sites will ignore the characters indicating difficulty.

With its 'basic' and 'extended' logic, the solver only makes deductions that are possible while looking at one point or one square at a time. When it looks at a square, it considers the number of the square (0, 1, 2, 3, or blank), the four sides of the square, and the eight external grid segments intersecting its four corners. For each square, it reads from, and writes to, only these twelve grid segments. The difference between 'basic' and 'extended' logic is that the former reads only lines and x's that have previously been written to the board (by the solver or user), and writes only lines and x's to the board, whereas the 'extended' mode extends the same logic by writing and reading pseudo lines and x's. If 'Use Patterns' is on, the solver also looks at the four neighbors of each square (there is a virtual row/column of blank squares on each side of the board) - the number and the four sides of each neighboring square.

Default	Use Patterns

In the four sections that follow, live puzzle boards are used for examples. After clicking on a link, click Next Bookmark or (Shift-)Right-click the Solve button for the next move. Here, with the mouse pointer positioned over a board - popup or embedded - the corresponding keys are Down-Arrow and 'V' (the solver will be at full strength), but the mousewheel will be most efficient; roll it down to see the next move. (The 'Solve based on Only One Solution' logic is turned off in the 'Use Patterns' section, except for one pattern that requires it.) Whenever the mouse pointer passes into a board, it will be reset to its initial position.

The solver doesn't use trial and error.