Objective / Rules - Complete the grid such that every row and column contains every number exactly once. - The symbols on the grid indicate neighbours (e.g. 1 >< 2, 3 >< 4, 2 >< 1). - Rule 1 - a symbol between = the numbers are neighbours. - Rule 2 - NOT a symbol between = the numbers are NOT neighbours.

Pencil Marks What is a pencil mark? Pencil marks can help you keep track of your own thinking, simply enter more than one digit/letter into each square, just like you might do if you were using pen and paper. The size of the digits/letters is smaller when there is more than one digit/letter in a square. A square might contain more pencil mark digits/letters than can be shown.

Keyboard Usage[only when your cursor is in the grid] Note: only when supported by your computer/device/browser/etc. A = auto-pencil marks (or click the check box). CTRL + arrows = move around the grid. SHIFT + number = highlight all squares with that number as a pencil mark [SHIFT+0 to clear].

Mouse Usage Clicking a neighbour symbol will change its colour, very useful when you know the symbol has been satisfied. "Show squares with a pencil mark..." highlights all squares with that number as a pencil mark.

Checking The system automatically checks that symbols and not symbols are being followed for single numbers (but doesn't check whether the numbers are correct).

If you click 'Check' the system will check for incorrect squares. If 'Show mistakes when checking' is checked they will be marked in red. There are two types of error checked for: 1. A single number that isn't the same as the solution. 2. Multiple numbers that don't contain the solution number.

Uniqueness Each puzzle has exactly one solution, which can be found using logic alone and no guesses are ever required.