Mason the Interposer

Tuck was six years old, with oversized boots and dust on his nose. He spent his afternoons sitting on an overturned mortar tub, watching Mason work.

One chilly Tuesday, Mason began building a strange stub of wall in the center of the market square. It was three feet long and four courses high. It held up no roof. It fenced in no pigs. It stood plain and awkward right between the fruit seller's basket and an old terrier named Barnaby.

Barnaby was a legendary guard dog. He spent every market day growling at anyone who stepped within five paces of the pears.

"That wall is silly," Tuck said, kicking his heels against the tub. "It doesn't do anything at all."

"Watch the dog," Mason said softly. He tapped a corner stone into place with his wooden mallet.

Tuck watched. Barnaby took two paces forward, lifted his nose, and found himself staring straight into a block of grey granite. On the other side of the wall, the pear basket sat entirely unguarded. A small rabbit walked up, picked out a ripe pear, and dropped two copper coins on the tray.

Barnaby never barked. He could not see the pears, so he did not know they needed protecting. He was still a ferocious guard dog. He was simply blind to the thing behind the stone.

"You blocked his look," Tuck declared, hopping off his tub. "You didn’t chase the dog away. You just put a stone in the middle of his eyes."

"I did," Mason said.

"But someone could knock your little wall down with a sledgehammer," Tuck pointed out. "It’s only four stones high."

"They could," Mason agreed, wiping sweat from his brow with a dusty sleeve. "And by the time they fetch the hammer, the rabbit has eaten the pear and gone home. A wall does not need to last forever. It only needs to stand until the work is done."

Tuck chewed on his lower lip. He walked around the stub of stone twice, testing the shadow with his paw.

"You got in the way on purpose," Tuck said slowly. "You caused an interference."

Mason stopped his trowel in mid-air. For Mason, pausing mid-stroke was like a clock skipping a tick.

"Interference," Mason repeated, testing the weight of the word like a new stone. "Getting in the way on purpose to cut a line. Yes. That is the exact shape of it."

Three winters later, a heavy damp fog rolled up the mountain path. With the fog came a stout rook wearing a green wool coat with polished brass buttons.

Captain Castle introduced himself with a formal bow that nearly knocked his waistcoat against his knees. He ran a school down in the valley where young animals learned the game of sixty-four squares.

"My students have a frustrating habit," the Captain explained, leaning against Mason’s wooden workbench. "When an enemy piece is guarded by another piece across the board, my pupils give up. They think a guarded piece is completely safe."

Mason scraped dried mortar off his blade. "And is it?"

"Not always," said Captain Castle. "They never think to step one of their own pieces straight into the line between the guard and its target. They don't realize that even if the guard captures the intruder, the defense is broken for one crucial move."

Mason smoothed a patch of grey cement along the wall. "I have spent my whole life placing stones across lines that other folks couldn't see. A board of sixty-four squares must be crisscrossed with lines."

"It is," said the Captain.

"Then I would like to cut a few," Mason said.

The chessboard, Mason discovered, was just a small town drawn in straight grey squares.

On a crisp October morning, Mason stood before his first class at the academy. Captain Castle set up a position on the demonstration board. A black rook sat undefended on an open square, but a black bishop watched it from six squares away down a long diagonal.

The young students leaned back in their chairs and sighed.

"You can't take the rook," said a young fox wearing a knitted green scarf. Her name was Bramble, and she kept a sharp pencil behind her ear. "The bishop is watching it down the row. If we take the rook, the bishop takes us."

Mason walked slowly to the board. He pointed his broad paw at the line of empty squares running between the bishop and the rook.

"The bishop guards the rook along this exact path," Mason said. His voice was calm and low. "Watch what happens when I drop a wall into the middle."

He lifted a white knight and placed it firmly on the square directly between the two black pieces.

"Now the bishop cannot see the rook," Mason said. "The line is broken."

"But they can just capture your knight!" Bramble protested, sitting up straight. "The bishop will eat your knight for dinner!"

"They can," Mason said mildly. "But they cannot do it right now. This move, they must make a hard choice. They can capture my knight and lose their rook, or they can save their rook and let my knight live. For one move, my wall stands. And one move is all I need."

He reached out and plucked the black rook off the board.

"The bishop was a fine guard," Mason added, dusting off the captured rook. "It simply could not guard through a solid wall."

Bramble stared at the board for a long moment. Then she pulled her notebook close and wrote carefully: A guard cannot guard through a wall, even a wall that falls next turn.

Captain Castle stood in the doorway, smiling quietly through his feathers.