# Making a grid deduction puzzle

When making a puzzle, what are the strategies in making it?

For most grid-deduction puzzles, simply putting numbers/dots/whatever in the grid simply doesn't cut it. However, sometimes you need to fit a certain constraint, such as the answer looking like a certain letter.

How do you go about it?

Please post different strategies as different answers, with the strategy name at the top and an explanation beneath. Try to include an example if you can. No need to spoilerise!

I am interested in both general strategies and those for specific puzzles, so post both!

## Bottom-up

In this strategy, you place objects in the grid that narrow down the solution set (without making the puzzle unsolvable) until there is exactly one solution.

This strategy works well on puzzles with rules that can be applied locally without affecting the global board too much.

Example: Slitherlink shaped in a P

First, let's do the end part, with two 3-0 configurations and the corner:

+ + +-+ +
|3|
+ +-+ +-+
0  |
+ +-+ +-+
|3|
+ + +-+ +

+ + + + +

+ + + + +


Now, let's do the bottom of stem with a 3 in the corner:

+ + +-+ +
|3|
+ +-+ +-+
0  |
+ +-+ +-+
|3|
+ + +-+ +

+ + + + +
|3
+-+ + + +


Now, since there needs to be a place for the bottom of the 3-0 configuration to come down to the 3 in the corner, we have this (not adding anything):

+ + +-+ +
|3|
+ +-+ +-+
0  |
+ +-+ +-+
| |3|
+ + +-+ +
|
+ + + + +
|3|
+-+ + + +


Then the other side of the loop must come up, and to determine the corner we can place another 3:

+-+ +-+ +
|3| |3|
+ +-+ +-+
|    0  |
+ +-+ +-+
| | |3|
+ + +-+ +
| |
+ + + + +
|3|
+-+ + + +


So the final puzzle looks like this:

+ + + + +
3   3
+ + + + +
0
+ + + + +
3
+ + + + +

+ + + + +
3
+ + + + +


## Top-down

In this strategy, you make the what you want to be the final puzzle, then knock out any information that you don't need.

This strategy works well on sudokus and other similar puzzles.

Example: 4x4 sudoku

13|24
42|13
--+--
31|42
24|31


You can remove an entire quarter:

13|
42|
--+--
31|42
24|31


And some more

13|
|
--+--
31|4
24|3


And some more

13|
|
--+--
31|4
24|3


Even more

13|
|
--+--
3 |4
4|3


And finally

13|
|
--+--
|4
4|3


(I don't think this can be reduced further)