Pencil mark notation: I've done a couple of things to help myself. First I drew a 9 x 9 table in MS Word obtaining 1/2 inch squares. I print up a dozen copies or so. I then copy the puzzle to the printed form, using a large colored pen.

After finding the obvious numbers, I set pencil marks in the following way: I visualize each square having nine positions , upper left, upper center, upper right, middle left, middle center, middle right, lower left, lower center, lower right.

I then work the grid for each square, marking a pencil dot (or small dash) in the nine positions for each number that might go into that square. For example, the left most column might have blank blank 2 blank 3 blank 7 blank 9 and the top row might hold blank 3 blank blank blank 1 blank 6 blank. So between the row and the grid, we accounted for 2, 3, 7, 9 vertically and 3, 1, 6 horizontally. We've accounted for 1, 2, 3, 6, 7, 9. The only numbers that might fit in the upper left blank, r1c1, are 4,5 and 8. I'd then put a dot in middle left, middle center and lower center.

This does take a bit of time to work through, but once you start to solve using forcing chains, the dots are easy to see and easy to erase as you fill in the puzzle. They are much quicker to place than writing little numbers, the standard pencil mark.

Yes, it would be a stinker on the 16 x 16 puzzles. kayjay40