Form a single closed loop out of the grid edges, in such a way that every number has exactly that many of the edges around it included in the loop.
When you have mastered the square grid, look in the Type menu for many other types of tiling, and in its Custom type for more still!
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click an edge to mark it as part of the loop (black), and again to return it to undecided (yellow). Right-click it to mark it as definitely not part of the loop (faint gray), and again to return it to undecided. Keep the button down and drag to do the same to every edge you pass over that looked the same as the first.
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the edge under it, and Space what a right-click does. Press Backspace or Delete to return the edge under the cursor to undecided; clicking it round gets there too.
Resting the mouse pointer on a line highlights every line joined to it, so you can see at a glance which pieces of the loop are already one piece.
When you draw a line, the app also rules out the edges that plain counting has already settled for you: the other edges at a dot that now has its two lines, and the other edges of a clue that now has as many lines as it asks for. Neither takes any working out — marking them is bookkeeping rather than solving — so they arrive with the line you drew, and a single undo takes the whole lot back together. If you would rather keep your own board, turn off Rule out edges that counting has already settled under Loopy preferences.
You can play entirely from the keyboard. Here you are marking the edges of the grid, so the cursor sits on a dot, where edges meet, and the arrow keys walk it from dot to dot along the grid’s edges. The edge you just walked along is the one your keys act on — think of a pen that inks where it has been. Walk, Enter, walk, Enter traces a loop; walk back over a marked edge and press Enter again to clear it.
The cursor is drawn as a green disc on its dot, with a green halo under the edge your keys will act on.
Two words are used throughout, here and in the hints, and they mean different things. An edge is one segment of the grid, between two neighboring dots: somewhere the loop could run, whether or not it does. A line is an edge you have marked as part of the loop. So an edge is the place and a line is the loop running through it — which is why a hint can say “this edge must be a line”, meaning the loop has no choice but to run along it, or “this edge can’t be a line”, meaning it must run somewhere else.
On harder boards you will often know something about a corner or a pair of edges before you know which edges are lines. Notes lets you write it down. Press the Marks key on the keypad, or P, to turn notes mode on, the same way as in every other puzzle that takes notes; a small pencil shows at the top right of the board while it is on, and pressing it again turns it off. (Loopy’s right-click is already how you rule an edge out, so it does not switch the mode here.)
From the keyboard, in notes mode:
Check & save compares your board with the puzzle’s answer. A line the loop does not use turns red, and an edge you ruled out that the loop needs gets a red cross, even if you have chosen not to show ruled-out edges. A note the answer contradicts turns red too.
Some red appears without asking, too: when the lines you have drawn break a rule, such as a dot with three lines or a closed loop that is not the only one.
Hint explains the next step rather than simply making it. It reasons from the lines you have drawn, the edges you have ruled out and your notes, so it carries on from wherever you are, as long as none of them is wrong; if one is, it asks you to fix the highlighted mistakes first.
Every dot takes either no lines or two.
Most steps are the rules at work: a clue that already has its lines, a clue with only as many edges left as it needs, a line with only one way to go on, a dot that already has two lines, or an edge that would close a loop too early.
One is worth learning, because it settles a whole clue at once. Suppose a clue needs all but one of its open edges: a 3 in a square, say, or a 2 in a square whose fourth edge is already ruled out. If two dots next to each other around it both already have a line, the edge joining them is out. Drawing it would fill both dots, ruling out the clue’s edge on the far side of each and leaving the clue short. With that edge gone, every other open edge of the clue must be a line.
On harder boards the reasoning turns on corners and pairs, and the hint writes each one down as a note before it uses it, just as you would. A note a step reasons from is already on the board. Two pairs that share an edge relate their other two edges, so a longer chain of pairs is written down one link at a time.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.