What is Shingoki?
Shingoki - Japanese for 'traffic light', and sold elsewhere under the name Semaphores - is a loop puzzle with a wonderfully simple deal: draw one closed loop through the centres of the cells so that it passes through every circle on the board. The circles are the traffic signals, and each one gives two pieces of information. Its colour tells you what the loop does at that spot: through a white circle the loop drives straight, at a black circle it turns. Its number measures the straight lines around it: add up the length of the two straight segments leaving the circle, counted in cell steps, and you must get exactly that number.
That is the whole game. The loop never crosses or branches, it does not need to visit every cell, and there is nothing else hidden in the rules. What makes Shingoki special is how those two small facts - a colour and a length - interlock. The colour fixes the shape of the loop at a point; the number fixes its reach. Solve a few boards and you start to feel the loop being measured into place, one signal at a time.
- Draw a single closed loop that travels horizontally and vertically through cell centres.
- The loop must pass through every circle on the board.
- White circle: the loop goes straight through it.
- Black circle: the loop turns at it.
- Gray circle: the loop may go straight or turn - the colour is withheld.
- A circle's number equals the combined length of the two straight segments leaving it, counted in steps.
- The loop never crosses itself, never branches, and may leave cells unused.
How to play Shingoki online
Click or tap the gap between two neighbouring cells to lay a piece of the loop, and click it again to remove it. The small dots mark the lattice your loop travels along; the circles are the clues it must collect. Keep connecting segments until they close into one loop that satisfies every circle - the board congratulates you the moment the last signal turns green.
Check marks anything that already breaks a rule - a branch, a white circle being turned at, a measurement that has grown past its number - without giving the route away. Hint removes a stray segment first and only then adds a correct one, Undo steps backwards, and Solution lays down the full loop when you would rather study a finished board than fight it.
- Tap between two cells to draw or erase a segment.
- Check highlights rule conflicts; it never spoils the route.
- Hint repairs your loop before it extends it.
- Backspace or Delete erases the segment you placed last.
- New puzzle builds a fresh board for the selected size and difficulty.
White circles are rulers, black circles are corners
The fastest way to get good at Shingoki is to stop reading the circles as decorations and start reading them as instruments. A white circle is a ruler: because the loop runs straight through it, its two arms lie on the same line, so the number is simply the length of the whole straight run it sits on. A white 5 says 'this straight stretch of road is exactly five steps long, and it turns at both ends'. You may not know yet whether the run is horizontal or vertical, but you know its exact length before you draw a thing.
A black circle is a corner with two measured arms. The loop turns on the circle, so one arm heads off in one direction and the other at a right angle, and together they total the number. A black 4 might split 1+3, 2+2 or 3+1 - but never 0+4, because every arm is at least one step long. Small numbers slam that range shut: a black 2 can only split 1+1, which means the loop turns on the circle and then turns again immediately on both sides. Three turns in three cells - it is the loudest clue in the game, and strong players draw it on sight.
- White n: one straight run of exactly n steps with the circle somewhere on it, turns at both ends.
- Black n: two perpendicular arms that sum to n, each at least 1 step.
- A black 2 is fully forced: both arms are 1, giving an instant zigzag.
- Two white circles on the same line with different numbers cannot share a run - the loop must turn somewhere between them.
- Two white circles sharing one run must carry the same number.
Strategy: let the walls do the measuring
Shingoki numbers live inside a box, and the box pushes back. An arm can never poke through the border, so every circle near the edge has its options trimmed before you start. Sit a white circle on the border row and its run cannot point out of the grid - it must lie parallel to that border, which immediately fixes the run's direction and often, with the number, its exact extent. A circle in the very corner of the grid is even blunter: the loop can only turn there, so a corner circle behaves as black no matter how it is painted, and its two arms run along the two borders.
The same wall arithmetic settles big numbers anywhere on the board. For each direction, count how many steps are available before the border; if the two best directions of a candidate orientation cannot add up to the circle's number, that orientation is dead. A white 9 on a 10x10 board has almost no freedom left once you count the room on each side of it. Work the loud clues first - black 2s, border circles, numbers close to the width of the grid - then let the loop they force grow toward the quieter ones. Each segment you draw gives some other circle a committed arm, and committed arms are what turn 'could be anything' into 'must be this'.
- An arm can never cross the border: count the room before believing an orientation.
- A white circle on the border row runs parallel to that border.
- A circle in a grid corner always acts as a turn, whatever its colour.
- Numbers near the grid's full width have very few legal placements - start there.
- Every placed segment commits an arm somewhere else; re-read nearby circles after each move.
Gray circles: the number still tells the truth
On hard boards some circles go gray. A gray circle withholds its colour - the loop may run straight through it or turn on it - but its number is exactly as honest as everyone else's: the two straight segments leaving it still sum to that value. Beginners treat gray circles as missing information. They are better read as a fork with two cleanly separated cases, because the straight reading and the turning reading usually reach into different parts of the board.
Run the two cases against the geometry. If the gray circle sits on the border, the straight reading must lie parallel to the wall, and the turning reading must use the one inward direction - check which of them can actually pay the number. If a neighbouring clue already commits one arm, the number tells you how far the remaining arm reaches in either reading, and frequently only one of them fits. Gray circles slow you down precisely because they make you do real deduction instead of pattern-matching - which is exactly why hard mode uses them.
- Gray = colour withheld; the number is still exact.
- Split into two cases: 'runs straight here' and 'turns here', and test each against the walls.
- A committed arm from a neighbouring clue often kills one of the two cases outright.
- If both cases force the same segment somewhere, draw that segment - it is true either way.
- Gray circles appear only on hard difficulty on this site.
The quiet rules that close the loop
Between the signals, Shingoki is governed by the bookkeeping every loop puzzle shares, and most stalled solves are unstuck by it rather than by the circles. Each cell the loop uses has exactly two connections - so the moment a cell has two segments, you may erase every other possibility around it in your head. A cell with one segment is a live end that must keep growing. And the loop is one loop: if connecting two ends would close a circle while circles elsewhere remain uncollected, that connection is forbidden, however natural it looks.
The 'every circle' rule has a sharp contrapositive that is easy to miss: if your partial loop ever walls a circle off - leaves it in a pocket the path can no longer enter without crossing itself - something earlier is wrong, even though no individual circle is complaining yet. Glance at the unreached circles every few moves and ask whether the loop can still get there. It is a two-second check that saves entire boards.
- Every used cell has exactly two connections - no branches, no dead ends.
- Two segments at a cell? Mentally seal the other two sides.
- Never close the loop while any circle is still outside it.
- A circle the loop can no longer reach proves an earlier mistake.
- Unused cells are fine; uncollected circles are not.
A traffic light from the internet age
Most of the loop puzzles on this site - Masyu, Slitherlink, Yajilin, Country Road - grew up on the printed pages of Japanese puzzle magazines. Shingoki did not. It is a young genre that found its audience online, spreading through dedicated puzzle sites in the late 2010s and arriving with two names already attached: Shingoki, from the Japanese word for a traffic signal, and Semaphores, the railway flag-talk that says the same thing in older technology. Both names describe the experience honestly - a board of lamps, each one regulating how the route flows past it.
Its closest ancestor is unmistakably Masyu, Nikoli's pearl-loop classic: white pearls that go straight, black pearls that turn. Shingoki keeps that beautiful colour grammar and bolts a measurement onto it, trading Masyu's local geometry rules for exact distances. The result feels different in the hands - less about spotting pearl patterns, more about arithmetic pressed against the walls of the grid - and it earned its place in the loop family rather than borrowing one. If puzzles have generations, Shingoki is what Masyu's grandchildren are playing.
Shingoki vs Masyu, Slitherlink and Country Road
All four are single-loop puzzles, and skills move between them, but each asks for a different instinct. Masyu gives you colours without numbers: a white pearl must be passed straight with a turn next door on at least one side, a black pearl turns with two straight steps on both arms - pure local geometry. Shingoki gives you colours with numbers, so the same pearls suddenly carry rulers, and the deductions become about distance. Slitherlink moves the loop off the cells entirely, drawing it along edges and counting how many sides of each square are used. Country Road and Yajilin add territory and shading to the loop instead of point clues.
Coming from Masyu, the adjustment is to trust arithmetic over pattern: in Shingoki it is normal to know a run's exact length long before you know where either of its ends lands. Going the other way, Shingoki practice quietly improves your Masyu and Slitherlink endgames, because all three share the closed-loop bookkeeping - degree-two cells, no early closure, one connected route - that decides boards after the clues run dry.
- Masyu: colour rules only, no numbers - local pearl geometry.
- Shingoki: colour rules plus exact arm lengths - geometry with a ruler.
- Slitherlink: loop on cell edges, clues count used sides.
- Yajilin and Country Road: loops combined with shading or rooms.
- Shared core: one closed loop, two connections per cell, no premature circles.
Grid sizes and difficulty levels
The 6x6 boards are the training ground: circles come thick, numbers stay small, and the loop is short enough that black 2s and border circles practically draw it for you. On 8x8 the runs stretch out, wall arithmetic starts doing real work, and you will meet your first boards where the middle stays ambiguous until two long arms finally collide. The 10x10 boards are full commutes - long straights, sparser signals, and routes that have to be grown patiently from several corners at once.
Difficulty changes the information, not just the size. Easy boards circle nearly half the loop and keep every colour visible. Medium thins the signals so you must bridge real gaps with loop logic. Hard thins them further and turns some circles gray, withholding colours exactly where they would have been most comforting. Whatever you pick, the generator solves every board before serving it and keeps adding circles until precisely one loop satisfies them all - so a stuck position never means a broken puzzle, only a deduction you have not found yet.
- 6x6 - learn the circle grammar and the wall arithmetic.
- 8x8 - balanced boards with genuine route planning.
- 10x10 - long runs and sparse signals for experienced solvers.
- Easy and medium show every colour; hard adds gray circles.
- Every puzzle is verified to have exactly one solution.