Given Cell
A digit pre-placed in the puzzle's initial state. Players cannot modify these cells.
A given (also called a clue) is a digit already placed on the grid at the start of the puzzle. Players cannot change these digits and must logically fill in the remaining empty cells. The number and placement of givens determine the puzzle's difficulty.
Relationship Between Givens and Difficulty
Generally, more givens make a puzzle easier. However, the number of givens alone does not determine difficulty; the placement pattern is equally important. The range of givens assigned to each difficulty label also varies between implementations, so you cannot work backwards from a count of givens to a difficulty level.
Minimum Number of Givens
It was proven in 2012 that the minimum number of givens for a Sudoku puzzle with a unique solution is 17. With 16 or fewer givens, a unique solution cannot be guaranteed. That said, 17 is only a lower bound, and having few givens does not automatically make a puzzle hard. Depending on how they are placed, even a puzzle with very few givens can sometimes be solved with basic techniques alone.
Reading the Givens Before You Start
When you open a grid, start by looking at how unevenly the givens are spread. Rows, columns, and boxes where the givens are packed together have fewer empty cells left, so they are the easiest places to pin a number down. Working there first fills in digits early and narrows the candidates across the whole grid. Regions where the givens are sparse can wait until later. When you get stuck, pick the digit that appears most often among the givens and scan the entire grid for that digit alone.
Points to Watch When Reading Givens
Givens are the digits supplied with the puzzle from the outset, and they are never rewritten while you solve. Because that is guaranteed, a given is always a clue you can trust. On the other hand, more givens do not always mean an easier puzzle. Even with a high count, an uneven placement can leave you without a single cell you can confirm in the opening. Treat both the number of givens and the difficulty label as rough guides, and judge by how the puzzle actually feels as you solve it.