Daily XV Sudoku
The extra rule
X joins two cells summing to ten. V joins two cells summing to five.
The letters come from Roman numerals, which is the whole mnemonic: X is ten and V is five.
Every X and V is shown, so an unmarked edge means that pair sums to neither five nor ten. That negative information does most of the work.
Only one pair sums to five in two ways worth noting, and no pair can sum to both five and ten, so a marked edge is never ambiguous.
Controls
Click a cell and type a digit, or use the pad. Space toggles pencil marks. Arrow keys move. Progress saves in this browser as you go, so you can come back to the same puzzle later today.
Solvable Without Guessing
Every puzzle here was generated and then handed to a solver that is only allowed techniques a person can use. If it could not finish the grid without a guess, the puzzle was thrown away rather than published. That also makes the answer unique, because every digit it placed was forced.
More to Play
There is also a kropki sudoku, a greater than sudoku, a even odd sudoku, a nonogram with hand drawn pictures, and 2048. All of them are on the games page, and the solvers for real objects are on the puzzle tools page.