Put each letter shown for this puzzle’s size and blank-count setting exactly once, plus the configured one or two blank cells, in every row and every column. A 4×4 puzzle always has one blank.
A letter outside the grid tells you the first non-blank letter visible when looking along its aligned row or column from that edge. Blank cells are skipped; the clue does not mean the adjacent cell must contain that letter.
A missing edge clue imposes no first-letter requirement from that edge. Clues on opposite ends of a line both apply independently.
The puzzle is solved when every row and column holds each letter exactly once and every edge clue is true. You do not have to mark the blanks — a line with all its letters placed has its blanks decided — but marking them helps you keep track.
The clue names the first letter you meet, not the first cell you meet. Any number of blanks may sit in front of it, so an edge cell clued A is either the A itself or a blank.
Example: A left clue of A on a line with one blank means either the first cell is A, or the first cell is the blank and the second is A.
The two clues on one line both apply, and each pins a letter to one of the few cells its own edge can still reach. Read them together before writing anything.
Example: On a 5×5 line with one blank, clued A from the left and B from the right, the A sits in cell 1 or 2 and the B in cell 5 or 4.
A line holds each letter once, so one letter can only be first from both edges if it is the only letter on the line — which never happens on a legal board. Two clues showing the same letter on one line is a sign you have mis-read which line a clue belongs to.
Example: A row clued A on the left and A on the right is impossible: the single A cannot be the first letter seen from both directions.
Every line owes exactly its letters plus its blanks. Once you have placed all the blanks a line is allowed, every remaining cell in it must hold a letter — and once the letters are all placed, the rest are blanks.
Example: On a 6×6 with two blanks, a row already showing both blanks has four letter cells left for the four remaining letters.
Every letter you place removes that letter from its row and from its column at once. After filling a cell, walk both lines it sits on before looking anywhere else.
Example: Placing a C in row 2 means no other cell in row 2, and no other cell in that column, can be a C.
If a clue’s letter is already fixed further along the line, every cell in front of it must be blank — and a line only has so many blanks to give, which often decides cells on the lines those blanks cross.
Example: A left clue of B whose B is known to be in cell 3, on a line with two blanks, makes cells 1 and 2 the blanks.
A cell in the first row and first column is seen by two clues at once. Where a top clue and a left clue disagree about what could sit there, the cell is usually a blank.
Example: A column clued A from the top and a row clued B from the left meet in one cell; it cannot be both, so unless one of them starts further in, it is blank.
A clue turns green as soon as the first letter you have committed on its line matches it, and red when a committed first letter contradicts it. Once a line holds all of its letters the leftover cells count as its blanks, so both of its clues settle even if you never marked them.
Example: A clue that goes red means the mistake is between that edge and your first letter on the line — undo back to it rather than hunting the whole board.