How to Play Trail: One Line Through Every Square
Trail's rules in full: one line from 1 to the highest number, through every square, in order, never crossing a wall. Plus a 5x5 puzzle solved move by move.
In Trail you draw one line through every square of the grid. It starts on the square marked 1, passes the other numbers in order (2, then 3, and so on) and ends on the highest number. The line moves up, down, left or right, never diagonally, never crosses itself or visits a square twice, and cannot pass through a thick wall. When every square is on the line and it ends on the last number, the puzzle is solved.
This guide is for complete beginners. It covers the rules, how to draw and undo in the app, the three ideas that give you your first moves, and a 5x5 puzzle solved step by step.
The rules of Trail
Trail has six rules, and the whole puzzle follows from them:
- Start on 1. The line begins on the square showing 1.
- End on the highest number. On a grid with numbers 1 to 7, the line finishes on 7.
- Pass the numbers in order. After 1 the line must reach 2 before 3, 3 before 4, and so on. It may not touch a number out of turn, not even to pass through it.
- Fill every square. Every square of the grid is on the line exactly once.
- Straight moves only. Each step goes to a square directly above, below, left or right. No diagonals.
- Walls block. A thick wall between two squares means the line cannot step from one to the other.
The fourth rule is what turns Trail from a maze into a puzzle. Plenty of routes lead from 1 to the last number; only one of them covers every square with the numbers in the right order.
The numbers are checkpoints, not step counts. In some path puzzles a number tells you how many steps into the path that square is. In Trail it only tells you the order. On a 5x5 grid there are 25 squares but only a handful of numbers (the example below has seven), so most of the line runs through blank squares, and you cannot tell from the numbers alone how long each stretch is. Hidato, Numbrix and Trail compared shows how much that one difference changes the solving.
If you have played LinkedIn's Zip, you already know these rules. Grandmaster Puzzles, which helps design Zip, describes it as drawing a path through all cells by moving horizontally or vertically, never crossing itself or a wall, starting on 1, passing the numbers in ascending order and ending on the highest.
Drawing, undoing and cutting the line
In Puzzle Picnic you draw the line by dragging from square to square, starting on 1. Two gestures fix mistakes:
- Drag back along the line to undo steps, one square at a time.
- Tap a square that is already on the line to cut it there. Everything after that square is removed, and you carry on from it.
There is no timer counting down. Your time is recorded, but nothing runs out, so you can stop and think at any point. If you are stuck, a hint (three free ones a day) explains the next logical step. A solve that used a hint is marked as assisted rather than clean.
Trail has four levels, and the grid grows with each one:
| Level | Grid | What changes |
|---|---|---|
| Easy | 5x5 | Open grid, more numbers as signposts |
| Medium | 6x6 | A bigger grid to plan across |
| Hard | 7x7 | Walls appear |
| Expert | 8x8 | More walls, and only the numbers it needs |
Every Trail puzzle in the app has exactly one solution, and logic gets you there, so you never have to guess.
Corners: where to start
A corner square has only two neighbours. If the corner is not the start or the end of the line, the line has to come in through one neighbour and leave through the other. So it must use both: you can draw the two short links through the corner before you know anything else.
That is why corners are the first thing to look at in any Trail puzzle. A 5x5 grid has four corners, and each one that holds no number, or holds a number other than 1 and the last, gives you two links for free.
The same reasoning works on any square with only two ways in or out. On Easy and Medium those are just the corners. On Hard and Expert, walls create more of them: a square on the edge of the grid with a wall on one side also has only two open neighbours, and it behaves exactly like a corner.
Never join numbers out of order
Two numbers can only be directly linked if they are consecutive. The line may step straight from 4 to 5, but never from 4 to 6, because it would skip 5. And a stretch of line that links 2 to 5 through blank squares is just as wrong: it skips 3 and 4.
This rule does most of the work in the opening, because it tells you which links are impossible. When a square could connect in three directions and one of them would join two numbers out of order, you are down to two directions, and if the square needs two links, it uses both.
No loops, no dead ends
The line is one path with two ends, 1 and the last number. Two consequences follow, and both are easy to check:
- No loops. If adding a link would close a stretch of line into a ring, that link is impossible. A ring has no ends, so it could never join up with the rest.
- No dead ends. Every square except the two ends needs one way in and one way out. If a blank square is left with only one open neighbour, the line could reach it but never leave. Avoid moves that leave a square like that, unless it is the square with the last number.
Dead ends are the most common reason a Trail line fails near the end. Before you sweep through a row, glance at the squares you are leaving behind and make sure each one still has two open neighbours.
A 5x5 Trail puzzle, solved
Here is a 5x5 puzzle, the size of an Easy Trail, with numbers 1 to 7. It has exactly one solution. Squares are named by row and column from the top left, so r3c5 is row 3, column 5.
Step 1: the corners. The top-right corner r1c5 is blank, so the line runs through it, linking r1c4 and r2c5. The bottom-left corner r5c1 is blank, so it links r4c1 (the 3) and r5c2. The bottom-right corner holds 5, which is neither the start nor the end, so 5 also needs both of its neighbours: it links r4c5 above and the 6 at r5c4.
Step 2: the 2 cannot reach the 5. The 2 at r3c5 needs two links, and it has three neighbours: r2c5, r3c4 and r4c5. But r4c5 is already linked to 5. Linking the 2 to it would join 2 straight to 5 and skip 3 and 4. So the 2 links to r2c5 and r3c4.
Step 3: full squares. r2c5 now has its two links (the corner above and the 2 below), so it cannot also link to r2c4. r4c5 has one link, to 5, and its only other open neighbour is r4c4, so it links there.
Step 4: out of order again. The line continues from the 2 through r3c4. That square's neighbours are r2c4, the 4 at r3c3, and r4c4. Linking to the 4 would skip 3. Linking to r4c4 would join the 2's stretch to the stretch through r4c4 and r4c5 that already ends at 5, skipping 3 and 4. So r3c4 links up to r2c4.
Step 5: no loop. r2c4 could now link to r1c4 or r2c3. But r1c4 is already joined to the corner r1c5, which runs down through r2c5 to the 2 and on to r3c4 and r2c4. Linking r2c4 to r1c4 would close that stretch into a ring. So r2c4 links to r2c3, and r1c4 must take its second link from r1c3.
After five steps, most of the right-hand side is drawn, and every move used one of three ideas: corners, numbers in order, and no loops. The rest of the grid falls the same way. The top row carries the line from 1 to the corner and down to 2. From 2 it snakes back along row 2 and down the left edge to 3, and from 3 through the bottom-left corner and up to 4. From 4 it drops into row 4, runs to the right edge and down to 5, and along the bottom row through 6 to 7.
Check the solution against the rules: it starts on 1, ends on 7, meets 2, 3, 4, 5 and 6 in order, covers all 25 squares once, and never steps diagonally.
Walls on Hard and Expert
Walls are thick lines between two squares that the trail cannot cross. In Puzzle Picnic they appear on Hard (7x7) and Expert (8x8), each one a short piece between two neighbouring squares. Walls look like they make a puzzle harder, but each one also removes options, and fewer options mean more forced moves.
Read walls the way you read corners. Count the open neighbours of each square: a square with only two open neighbours, because of the grid edge, walls or both, must use both of them unless it is the start or the end.
In other path puzzles, longer walls can form corridors with only one way through, or split the grid into areas that the line must enter and leave through a narrow gap. Walls in path puzzles works through these patterns in detail.
Frequently asked questions
Does the line have to go through every square in Trail?
Yes. A Trail puzzle is only solved when the line covers every square of the grid exactly once and ends on the highest number. Reaching the last number with squares left over does not count, so if you arrive early, something earlier in the line needs to change.
Can the line go diagonally?
No. In Trail the line moves only up, down, left or right. Diagonal steps are allowed in some other number-path puzzles, such as Hidato, but not in Trail.
Is Trail the same as LinkedIn's Zip?
The rules are the same. Both ask for one path through every square, starting on 1, passing the numbers in order, ending on the highest number, moving only horizontally or vertically, and never crossing itself or a wall. The puzzles themselves are different, and Trail offers four levels from 5x5 to 8x8.
What do the numbers mean in Trail?
The numbers are checkpoints that set the order of the line, not counts of steps. The line must reach 2 after 1, then 3, and so on, but any number of blank squares can lie between two numbers.
Do I ever have to guess in Trail?
No. Every Trail puzzle in Puzzle Picnic has exactly one solution that logic can reach. When you are stuck, check the corners, look for links that would join numbers out of order or close a loop, and make sure no blank square is left with a single way in.
Where to go next
Start with a few Easy grids and work corners first every time. Once the three ideas in this post feel natural, the path puzzle strategies guide adds the next layer: dead ends, chessboard parity and estimating the distance between numbers. If you are curious why one-line puzzles are such an old idea, the maths behind one-line puzzles goes back to William Rowan Hamilton's Icosian game of the 1850s. You can play a Trail puzzle in your browser on the Trail page, and the best logic puzzles for beginners compares Trail with the other five games.