Welcome to the interactive Calibron 12 puzzle (invented by Theodore Edison in 1933)! Your goal is to pack all 12 rectangular blocks into the 56×56 board with zero wasted space.

How to play:

• Click (or drag) any piece from the tray onto the board to place it into the next open Top-Left Corner (marked with a highlighted pink dot on the board).
• Right-click (or long-press on mobile) any piece in the tray to flip its orientation by 90° before placing, or click a placed piece on the board to undo back to that step.
• Once you have placed at least 4 pieces, the "Check" button unlocks! Click it to ask the backend CSP solver to verify whether a valid completion exists from your partial placement; if one is found, you can choose whether to reveal the full solution or keep solving on your own.

• Animate solve attempt renders the solver's intermediate piece placements and backtracks on the board while checking your progress (which may give away the solution as pieces are placed), returning to your placed pieces once finished.
• When Fast execution is enabled (available when animating), the solver runs un-throttled on the backend. Leaving it off steps through the search at a slower pace so each placement attempt is easy to follow.
• Show top-left corners toggles rendering of the candidate placement corners on the board, which are maintained incrementally by splitting overlapping Maximal Empty Rectangles into at most 4 sub-rectangles.

You can share a partially-filled puzzle either by clicking the link button below (right next to the "?"), or simply by copying the URL, which is updated automatically as you place and rotate blocks.

I've included a couple of links below, feel free to reach out to me with ideas, suggestions or other feedback.