- Move your cursor over the "New Puzzle" menu item and select a skill level from the dropdown menu.
- Fill in blanks with the Pen (select a cell, then click on the number you want to enter, or enter the number via the keyboard).
- If you are not 100% sure of a number, you can make a note of it in the cell using the Pencil. To do this, click the "Pencil" icon to be placed in "Pencil mode".
- You may enter as many pencil marks as you like in each cell.
- To change back to "Pen" mode, click the "Pen" icon. You can also switch between pen and pencil modes quickly by pressing the "P" key. (See tips below.)

- Use the arrow keys to navigate around the grid.
- Use the number keys to enter a number in the highlighted cell.
- Press the space or Delete key to remove any numbers from the highlighted cell.
- Press the "P" key to toggle between Pen and Pencil.
- Press the "U" key to undo last input.
- Hold
**Shift**key down to enter in pencil (when in Pen mode).

- Click the Undo icon (yellow arrow) to undo last input. You can undo all the way back to the beginning.
- Holding the
**Shift**key when clicking on a number in the input panel will pencil that number in. You may find this more convenient than toggling between the Pen and Pencil icons. - To remove just one pencilled number in a cell, enter that number again.
- To remove all pencil numbers in a cell, either click the eraser button or simply "pen" the final choice in.
- To remove all pencil nubers from all cells, use the menu option under "Edit".
- There is an alternate mode of number input via the mouse called "Active Number Mode" (see below).

To change the active number, select a new one from the Number Pad or scroll the mouse wheel down/up to increase/decrease the number.

In the Active Number mode, all pencil functionality remains the same as does the ability to use the keyboard.

**Note: Initially, there is only one new puzzle per level per day.**

Preferences are remembered and will reload from what was last set on that computer.

__Rule of k__

The "Rule of 1" means that if a cell has only one possible value, then all other cells in the same region (row/col/block/hyperblock) cannot have that value (uniqueness of digits).

The "Rule of k" is just an extension of this rule which says that if there are k cells in a region that each contain exactly the same k different possible values, then no other cell in that region can contain any of those k values. Therefore we can eliminate those possibles from the other cells in the region.

For example, if two cells in a row both contain the possibles "34" only, then we know that one of the cells is a "3" and the other "4" (although we don't know which is which). But it does mean that no other cells in the same row can be a "3" or "4" so we can eliminate those numbers as possibles for them. The same logic can be applied to 3 cells with the same 3 possible values, and so on.