The case notes
How a lockout line works
Diamond end digits differ by at least four. Digits on the line are strictly outside the interval between the ends. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Diamonds differ by 4 or more
A lockout line has diamonds at its ends. The two diamond digits must differ by at least four.
Middle: below 4 or above 8
These ends are four and eight. The cells between them on the line must lie outside that range: below four or above eight.
The 9 lies outside the range
Nine works in the first inner cell because it is above eight. Four, five, six, seven and eight are all locked out of the middle.
Only 3 remains
The next inner cell must be one, two or three; nine is already in its row. One and two are also used there, leaving three.
Gap first. Outside next.
First check the diamond gap. Then keep every inner digit strictly outside the two end values. Other Sudoku rules can settle which one fits.
Read the transcript
A lockout line has diamonds at its ends. The two diamond digits must differ by at least four. These ends are four and eight. The cells between them on the line must lie outside that range: below four or above eight. Nine works in the first inner cell because it is above eight. Four, five, six, seven and eight are all locked out of the middle. The next inner cell must be one, two or three; nine is already in its row. One and two are also used there, leaving three. First check the diamond gap. Then keep every inner digit strictly outside the two end values. Other Sudoku rules can settle which one fits.
