The square is back to normal now. One side note: if you like to use your keyboard's number pad to enter numbers, the * key will act just like the L key.

The next page will show you how to use undo and redo.

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Oops - you tried to select a different square. I reselected the locked square for you.

Press the L key again to unlock the square.

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Notice that none of the numbers changed, and the lock flashed red.

At some point, you might want to unlock a square. You can do that by pressing the L key again.

To unlock a square, simply press the L key again.

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Oops - you tried to select a different square. I reselected the locked square for you.

Try to change some numbers in the locked square.

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The square is now locked. When you try to change a locked square, the lock image will flash red to remind you that the square is locked.

Try to change some numbers in the locked square.

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Depending on your style of solving, it can sometimes be helpful to mark squares, or to temporarily lock the numbers in a cell. You can do this with the lock feature.

When a square has been locked, a grey lock image will appear in it. You won't be able to make any changes to that square as long as it's locked.

Pick a square in the grid to the right and press the L key to lock it.

And that's all there is to know about undo and redo.

The next chapter will tell you about the options you can use to customize your game.

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Just like undo goes back to the beginning, redo will go back to the end (as long as you haven't made any new changes).

Press Ctrl + Y a few more times.

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What if you discover you've undone too many changes? You can reverse the process by redoing the changes. They'll all be done again, in the same order you did them originally.

Important note: Be very careful when undoing changes! If you use undo, then make any new changes to the grid, you won't be able to redo the old changes anymore.

Press Ctrl + Y to redo the last thing you undid.

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You can go even farther than the last change. In fact, you could undo your way all the way to the original, blank grid.

Press Ctrl + Z a few more times.

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When you press the undo key, the latest change you made will be reversed. If you added a number to a square, it'll be removed. On the other hand, if you just removed a number, it'll be added back to the square.

Press Ctrl + Z to undo the last thing you did.

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At some point, you may discover that you've made a mistake somewhere along the way when solving a puzzle. While techniques for searching for mistakes are beyond the scope of this help page, you might find the undo and redo features helpful when you need to back up a few moves.

First, let's set up some numbers you can back up through. Enter at least four or five numbers in the grid to the right.