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A 6×6 letter logic puzzle. Every row and column holds A, B, C and D exactly once, with two squares left empty, and the letters around the edge tell you which letter you meet first when you look in from that side. One solution per board, found by logic alone — beat your best time.
The board is a 6×6 grid surrounded by a ring of clue spaces. Fill it so that every row and every column contains each of the letters A, B, C and D exactly once. The remaining two squares of every row and every column stay empty.
A letter outside the grid tells you the first letter you meet when you look along that row or column from that side, skipping over empty squares. A B to the left of a row means that, reading the row from the left, the first letter is B; anything before it is empty. Not every side has a clue, and a missing clue tells you nothing.
Press Start and the clock begins. Tap a square to select it, then tap A, B, C or D on the pad. Use × to mark a square you know is empty, and ⌫ to clear it. Tapping the same choice again also clears it. Turn on Notes to jot small pencil marks of the letters that are still possible; placing a letter removes it from the notes in its row and column.
The clues check themselves as you go. When the first non-empty square seen from a clue holds the right letter, the clue is underlined; when it holds a different letter, the clue turns red and is struck through. A letter used twice in a line, or more than two × marks in a line, turns those squares red. The puzzle is complete the moment all 24 letters are in their places — the empty squares fill in by themselves.
Your score is the time you took, and lower is better. The clock is real time, so it keeps running if you switch apps. Each new board is drawn from a large bank and shown with its letters renamed and the grid turned or flipped at random, so you will rarely see the same picture twice. On a keyboard you can type A–D, use X, 0 or Space for empty, the arrow keys to move, Backspace to erase and N to toggle notes.
Every line has only two empty squares, which makes the clues very tight. The first letter seen from a side must sit in one of the first three squares from that side — at most two empty squares can come before it. So a B clue on the left of a row puts B in the first, second or third square, and everything in front of it must be empty. If the first square of that row already holds some other letter, the clue is broken.
Corners are the classic starting point. The corner square of the grid is the first square seen from two clues at once, one for its row and one for its column. A letter in the corner would be the first letter for both of them, so if the two clues name different letters, the corner must be empty. The same reasoning works one step further in: when the corner is empty, look at the next squares along each line.
Count the empties. As soon as a line has its two empty squares marked, every other square in it is a letter, and the clues on both ends then have to be satisfied by the letters at the very ends of the line. Conversely, when a line already has all four letters placed, the remaining two squares are empty and you can mark them with × to help the crossing lines.
Use the opposite clues together. In a row with A on the left and C on the right, A is in the first three squares, C is in the last three, and the other two letters must sit between them — anything outside that pair would be seen first instead. Notes are useful when two letters compete for the same square. Easy as ABC, also published as End View, is a regular in puzzle championships and puzzle magazines; boards here come from a bank in which every board has a single solution that can be reached line by line, without guessing.