The dominoes

All polyominoes of order 2 — figures built from 2 cells. Edge-joined unit squares of the square lattice — the classic polyform family, home of the twelve pentominoes.

✓ 15/15 verification checks passed

• PASS · all forms have n cells
• PASS · all forms connected
• PASS · fixed forms distinct
• PASS · orbit count: free -> fixed
• PASS · orbit count: free -> one-sided
• PASS · orbit count: one-sided -> fixed
• PASS · free <= one-sided <= fixed
• PASS · canonical form idempotent
• PASS · canonical form group-invariant
• PASS · encoding round-trips
• PASS · free forms expand to the fixed set
• PASS · free forms expand to the one-sided set
• PASS · matches A000105 (free)
• PASS · matches A000988 (onesided)
• PASS · matches A001168 (fixed)

drawing…

Free 1 polyominoes

Two figures are the same free form if one can be turned into the other by translation, rotation or reflection. This is the usual count — the one that gives 12 pentominoes.

One-Sided 1 polyominoes

One-sided forms may be translated and rotated but not flipped over, so a chiral figure and its mirror image are counted separately.

Fixed 2 polyominoes

Fixed forms may only be translated. Every distinct rotation and reflection is counted as its own figure.