A diagonal is a straight line of same-colored squares running at forty-five degrees across the chessboard, touching corner to corner of each square it passes through — for example, the line a1, b2, c3, and onward to h8. Diagonals are the third direction of chess geometry, alongside the horizontal ranks and vertical files, and they are the exclusive roads of the bishop, half the power of the queen, and the way every pawn captures. Learning to see diagonals as clearly as you see straight lines is, in my experience as a coach, one of the real dividing lines between a beginner and an improving player.
What Is a Diagonal?
Look at any square of the chessboard — say e4 — and trace the squares that touch it only at the corners: d3 and f5 one way, d5 and f3 the other. Extend those corner-to-corner steps in a straight line and you have drawn the two diagonals passing through e4. Unlike ranks and files, which all have exactly eight squares, diagonals come in sizes: the two long diagonals, a1-h8 and h1-a8, have eight squares each, while the tiny diagonals in the corners — a2 to b1, or g8 to h7 — have only two. In total the board contains twenty-six diagonals, thirteen leaning each way.
Diagonals are usually named by their endpoints: the a1-h8 diagonal, the b1-h7 diagonal, the a2-g8 diagonal. And here is their first deep property: because a diagonal step always moves one square across and one square up, every square on a given diagonal is the same color. The a1-h8 diagonal is entirely dark; h1-a8 is entirely light. This is not a decoration — it is the geometric fact from which an enormous amount of chess strategy grows, as we will see.
If ranks are the board's latitude and files its longitude, diagonals are the slanted trade routes that only certain pieces know how to travel. A player who watches only the straight lines sees half the board's road map.
The Pieces That Live on Diagonals
Three of your six piece types care about diagonals, and one of them cares about nothing else. The bishop moves exclusively along diagonals, any number of squares, which is why a bishop that starts on a light square remains on light squares for its entire life — its whole world is the thirteen light-squared diagonals. The queen combines rook and bishop, so half of her enormous range is diagonal; the deadliest queen maneuvers in chess are frequently diagonal ones, sliding from a quiet square into an attacking post the opponent forgot was connected. And the humble pawn, which walks straight ahead, captures one square diagonally forward — meaning every capture that pawns make, and every pawn chain that locks a position, is diagonal architecture. Even the king steps diagonally when it pleases, and in king-and-pawn endgames the diagonal walk is a famous resource: a king moving from a1 toward h8 by diagonal steps covers ground in two directions at once, the geometric trick behind many studies where the king seems to be in two places at the same time.
When I introduce the pieces to new students, I make them physically trace diagonals with a finger, because the untrained eye simply does not register slanted lines the way it registers rows and columns. Nearly every beginner blunder of the type "I never saw that bishop" — and the hanging piece it costs — is a diagonal that went unseen. If you are at that stage, our guide on chess for beginners builds this board vision step by step.
The Long Diagonals: Prime Real Estate
Not all diagonals are equal, and the two eight-square monsters — a1-h8 and h1-a8 — are the most valuable slanted lines on the board. They pass through the four central squares, they connect corner to corner, and a bishop or queen installed on one of them can influence both wings simultaneously.
Chess opening theory has an entire architectural style built around them: the fianchetto, from the Italian for "little flank," where you play g3 and Bg2 (or b3 and Bb2, or as Black g6 and Bg7) to post the bishop on the long diagonal. Entire opening families — the King's Indian, the Grünfeld, the Catalan, the London's rival systems with b3 — are debates about whose long-diagonal bishop will matter more. A fianchettoed bishop rakes through the center toward the enemy corner, and part of its job is prophylactic too: it stands guard over the squares in front of its own castled king. That double duty explains a rule every King's Indian player learns in tears: trading off your fianchetto bishop leaves the squares it guarded — g7, h6, f6 — chronically weak, and an attack on those squares often follows. The interactive lessons in Learn Openings show these fianchetto structures move by move.
The long diagonals also produce the most photogenic finishes in chess: a queen and bishop lined up on b2 and c3, both staring at a castled king on g8, is a wrecking ball. Which brings us to attacking geometry.
Diagonals as Attacking Highways
If files are the rook's invasion routes, diagonals are the light cavalry's. A remarkable share of standard attacking patterns are named, at bottom, after a diagonal.
The b1-h7 diagonal (and its mirror b8-h2) is the most famous attacking road in chess: a bishop on d3 aims through it at h7, the pawn beside Black's castled king, and the Greek gift sacrifice — Bxh7+ followed by Ng5 and Qh5 — is the classical demolition that every attacking player must know from both sides. The a2-g8 diagonal points at f7, the weakest square in Black's starting camp, defended only by the king; a bishop on c4 eyeing f7 is the engine of the Scholar's mate that catches beginners, but also of perfectly respectable attacks in the Italian Game — see our guide on how to play the Italian Game for the grown-up version. Even the fastest checkmate possible, the Fool's mate, is a diagonal story: White weakens the e1-h4 diagonal with f3 and g4, and the queen arrives on h4 to end the game in two moves.
Diagonals are also the natural home of two fundamental tactics. A pin along a diagonal — bishop on g5 tying the knight on f6 to the queen on d8 — is often the very first positional tactic a player learns, and the skewer is its reversed twin, forcing the valuable piece to move and collecting what stood behind it. When a queen lines up in front of or behind a bishop on the same diagonal, you have a diagonal battery, and batteries on the long diagonal or on b1-h7 are the standard heavy machinery of kingside attacks.
The practical habit I teach: whenever any piece lands on a new square, trace both of its diagonals all the way to the edge before you do anything else. Most "sudden" attacks were visible for moves along a slanted line nobody followed to its end.
Light Squares, Dark Squares, and the Strategy of Color
Now for the deepest consequence of the fact that a diagonal keeps one color. Because each bishop is confined to half the board's squares, diagonal strategy and color strategy are the same subject.
When the pawns lock into a chain — itself a diagonal of pawns — they permanently occupy squares of one color and permanently concede the diagonals of the other color. A bishop stuck behind its own pawn chain, shuffling on blocked diagonals, is the classic bad bishop; its happy twin, the good bishop, patrols the open diagonals the pawns left free. When one player has no bishop of a given color and their pawns cannot cover those squares either, the opponent may seize an entire network of same-colored diagonals — a color complex — and pieces settle onto every weak square in the network as if they owned it. Some of the most one-sided grandmaster games ever played are light-square strangulations in which the losing side's pieces were never actually captured, just made irrelevant, square by same-colored square.
This is also the logic behind the bishop pair advantage: two bishops together cover diagonals of both colors, so they have no complex to be weak on, and in open positions they sweep the board like crossed searchlights. In my book Chess Structures I return to this theme constantly, because you cannot evaluate a single pawn chain without asking which diagonals it opens, which it closes, and which bishop just got better or worse.
One warning from the endgame: diagonals even decide draws. In the notorious wrong bishop ending, a bishop plus rook-pawn cannot win if the bishop's diagonals never touch the promotion corner — the defending king sits in the corner of the color the bishop can never reach, and the full point evaporates. Color, corner, diagonal: one geometric fact, half a point.
How I Train Diagonal Vision
Diagonal blindness is curable, and the cure is repetition with feedback. Three exercises I assign, in increasing order of difficulty.
First, edge-tracing: on any position, pick every bishop and queen and name the terminal square of each of its diagonals, through and past any blockers. You are training the eye to complete lines instead of stopping at the first obstacle — essential for spotting discovered attacks, where a blocker steps aside and the diagonal behind it fires.
Second, capture-scanning for pawns: before every move, name each enemy pawn's two diagonal capture squares. It sounds absurdly basic; it eliminates the single most common beginner blunder I see, pieces placed one diagonal step in front of an enemy pawn.
Third, color audits: in quiet positions, ask which color of squares each side is strong on, and which bishop — yours or theirs — would you trade tonight if you could. This question, asked every few moves for a few months, quietly installs the entire strategy chapter above.
Diagonals at ChessMind AI
Diagonal patterns run through everything we built. The puzzle collection is saturated with diagonal tactics — pins, skewers, batteries, Greek gift demolitions — and solving them daily is the fastest way to make slanted lines jump off the board at you. Our miniatures trainer replays short decisive games, which are disproportionately diagonal catastrophes around f2 and f7, so you learn the opening's dangerous diagonals by watching them claim victims. The positional puzzles train the slower skills of this article — good bishop against bad, seizing a color complex, choosing the right pawn chain — and our visualization exercises drill exactly the line-tracing muscle that diagonal vision depends on, since slanted lines are precisely the ones the mind's eye loses first.
Frequently Asked Questions
How many diagonals are on a chessboard?
Twenty-six: thirteen slanting in each direction. They range in length from the two-square corner diagonals (like a2-b1) to the two eight-square long diagonals, a1-h8 and h1-a8. Every square lies on exactly two diagonals, one of each slant.
Why are all squares on a diagonal the same color?
Because each diagonal step moves one file across and one rank up or down, and the board's coloring alternates with every single-square step in one direction. Moving in both directions at once lands you back on the starting color. This is why each bishop patrols only half the board for its entire life.
Which piece moves only on diagonals?
The bishop. It slides any number of squares along a diagonal but can never leave the color it started on. The queen moves diagonally as well (in addition to ranks and files), pawns capture one square diagonally forward, and the king can step one square diagonally.
What is the long diagonal in chess?
Either of the two eight-square diagonals connecting opposite corners: a1-h8 (dark squares) and h1-a8 (light squares). They cross the center of the board, which makes them the most powerful diagonals to control, and the fianchetto — developing a bishop to g2, b2, g7, or b7 — is the standard method of claiming one.
Why is the a2-g8 diagonal so dangerous for Black?
Because it ends on f7, the one square in Black's starting position defended only by the king. A White bishop on c4 or b3 aims down this diagonal at f7 from the opening's first moves, which powers everything from beginner traps like the Scholar's mate to main-line attacking systems in the Italian Game and Ruy Lopez. Black's mirror weakness is f2, targeted along a7-g1.