You are given a grid with several blocks. Fill every blank cell with a line, which connects two cell edges and goes horizontally or vertically through the center of the cell. The lines must follow these rules:
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 a square to place a vertical line in it (click again to turn it horizontal, and again to clear it). Right-click it to place a horizontal line in it (again to turn it vertical, and again to clear it).
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the square under it, and Space what a right-click does. Press 1 to place a vertical line in it; clicking it round gets there too. Press 0 or 2 to place a horizontal line in it; right-clicking it round gets there too. Press Backspace or Delete to clear it; clicking it round gets there too.
You can also drag horizontally or vertically across the squares to draw a line that way through each of them; a drag that starts along a line already there clears the lines it passes over instead. Hold Shift or Ctrl while moving the cursor to draw a line through the square it leaves and the square it enters (Shift across the way it moves, Ctrl along it), or both to clear them.
A line is connected to a block when it runs into it: a horizontal line in the cell to its left or right, or a vertical line in the cell above or below. A line lying alongside a block — a vertical line beside it, or a horizontal line above it — does not connect to it, and can never come to. That is why a block can run out of usable sides long before its number is met.
This puzzle type was invented by Nikoli, and is known as Tatebo-Yokobo.
More information: https://www.janko.at/Raetsel/Tateboo-Yokoboo/index.htm
Hint explains the next step rather than simply making it. It reasons only from the numbers and the lines you have placed, so it carries on from wherever you are, as long as none of your lines is wrong; if one is, it asks you to fix the highlighted mistakes first.
Every step rules one direction out and so leaves the other. The hint names the square by its number when it has one (“this 2 must be vertical”), and a block’s number as “the 2 block”. The reasons it gives are the rules: a line that would grow too long for its number, one that would leave a number too little room to reach its length, one that would join two numbers into a single line, one that would run into a block that already has all its lines, and one that would close off a side a block still needs, which the hint calls an open side. When one number rules out several squares, the hint walks through them one at a time as a single step.
On an Unreasonable board there comes a point where no square has a direction that breaks a rule at once. The hint then says that nothing further follows by deduction instead of choosing for you. Save your position and try a line in a square beside a number: if it is wrong, a few forced squares later some number has no way left to be met. The hint carries on from whatever you decide.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.