Draw a single closed loop by connecting together the centers of adjacent grid squares, so that some squares end up as corners, some as straights (horizontal or vertical), and some may be empty. Every square containing a black circle must be a corner not connected directly to another corner; every square containing a white circle must be a straight which is connected to at least one corner.
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).
Drag from square to square to draw the loop along the edges you pass. A drag only adds: passing back over a square it has already drawn through does nothing, and it carries on once you return to where it reached, while dragging back onto the square you started from closes the loop. Undo takes a stroke back. A drag along a stretch of loop you have drawn and on past its end carries that stretch on, and a drag that runs only along lines you have drawn takes them up. A line stops where it would give a square a third line, or where you have crossed the edge out. You can also click near the edge between two squares to draw or remove a line there.
Right-click an edge to mark it with a cross indicating that you are sure the loop does not go through it, and keep the button down and drag to cross every unmarked edge you pass over.
The keyboard can also be used. The arrow keys move a cursor. Press Enter to start drawing, move the cursor along the path you want (moving back along it takes a step back), and press Enter again to draw or undraw it; Space, Escape, Backspace or Delete abandons the path instead. Holding Ctrl while pressing an arrow key draws or undraws the line in that direction and moves along it, holding Ctrl and Shift together does the same without moving, and holding Shift alone marks or unmarks a cross on that edge.
Hint explains the next step rather than simply making it. It reasons only from the lines you have drawn, the edges you have crossed out and the pearls, so it carries on from wherever you are, as long as none of those is wrong; if one is, it asks you to fix the highlighted mistakes first.
Most steps are the rules at work. A square the loop enters has to be left by another edge, and a square with nowhere else to go stays empty. A black pearl turns, so of each two opposite edges it uses exactly one, and its line then runs straight through the next square on both sides. A white pearl runs straight through, so it takes both edges on one side-to-side or up-and-down line, and its loop must turn in a square beside it.
A few are worth learning, because they settle a pearl from farther away:
A square that already has its two lines needs no crosses on its other edges, so the hint never asks you to draw them, and it counts such an edge as closed when it reasons.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.