In geometry, a Cairo pentagonal tiling is a tessellation of the Euclidean plane by congruent convex pentagons, formed by overlaying two tessellations of the plane by hexagons and named for its use as a paving design in Cairo. It is also called MacMahon's net after Percy Alexander MacMahon, who depicted it in his 1921 publication New Mathematical Pastimes. John Horton Conway called it a 4-fold pentille.

Infinitely many different pentagons can form this pattern, belonging to two of the 15 families of convex pentagons that can tile the plane. Their tilings have varying symmetries; all are face-symmetric. One particular form of the tiling, dual to the snub square tiling, has tiles with the minimum possible perimeter among all pentagonal tilings. Another, overlaying two flattened tilings by regular hexagons, is the form used in Cairo and has the property that every edge is collinear with infinitely many other edges.

In architecture, beyond Cairo, the Cairo tiling has been used in Mughal architecture in 18th-century India, in the early 20th-century Laeiszhalle in Germany, and in many modern buildings and installations. It has also been studied as a crystal structure and appears in the art of M. C. Escher.

The union of all edges of a Cairo tiling is the same as the union of two tilings of the plane by hexagons. Each hexagon of one tiling surrounds two vertices of the other tiling, and is divided by the hexagons of the other tiling into four of the pentagons in the Cairo tiling. Infinitely many different pentagons can form Cairo tilings, all with the same pattern of adjacencies between tiles and with the same decomposition into hexagons, but with varying edge lengths, angles, and symmetries. The pentagons that form these tilings can be grouped into two different infinite families, drawn from the 15 families of convex pentagons that can tile the plane, and the five families of pentagon found by Karl Reinhardt in 1918 that can tile the plane isohedrally (all tiles symmetric to each other).

One of these two families consists of pentagons that have two non-adjacent right angles, with a pair of sides of equal length meeting at each of these right angles. Any pentagon meeting these requirements tiles the plane by copies that, at the chosen right angled corners, are rotated by a right angle with respect to each other. At the pentagon sides that are not adjacent to one of these two right angles, two tiles meet, rotated by a 180° angle with respect to each other. The result is an isohedral tiling, meaning that any pentagon in the tiling can be transformed into any other pentagon by a symmetry of the tiling. These pentagons and their tiling are often listed as "type 4" in the listing of types of pentagon that can tile. For any type 4 Cairo tiling, twelve of the same tiles can also cover the surface of a cube, with one tile folded across each cube edge and three right angles of tiles meeting at each cube vertex, to form the same combinatorial structure as a regular dodecahedron.

The other family of pentagons forming the Cairo tiling are pentagons that have two complementary angles at non-adjacent vertices, each having the same two side lengths incident to it. In their tilings, the vertices with complementary angles alternate around each degree-four vertex. The pentagons meeting these constraints are not generally listed as one of the 15 families of pentagons that tile; rather, they are part of a larger family of pentagons (the "type 2" pentagons) that tile the plane isohedrally in a different way.

Bilaterally symmetric Cairo tilings are formed by pentagons that belong to both the type 2 and type 4 families. The basketweave brick paving pattern can be seen as a degenerate case of the bilaterally symmetric Cairo tilings, with each brick (a ${\displaystyle 1\times 2}$ rectangle) interpreted as a pentagon with four right angles and one 180° angle.

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  Type 2 Cairo tiles have non-adjacent complementary angles, with the same two adjacent side lengths
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  Type 4 tiles have non-adjacent right angles between pairs of equal-length sides
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  Bilaterally symmetric tilings (belonging to both types) use tiles with non-adjacent right angles and four equal edges

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  Type 2 Cairo tiling, with coloring showing reflected and non-reflected tiles
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  In a type 4 Cairo tiling, the pentagons can be bilaterally symmetric even when the tiling isn't
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  The basketweave, a degenerate bilaterally symmetric tiling, with non-degenerate tiling overlaid

It is possible to assign six-dimensional half-integer coordinates to the pentagons of the tiling, in such a way that the number of edge-to-edge steps between any two pentagons equals the L distance between their coordinates. The six coordinates of each pentagon can be grouped into two triples of coordinates, in which each triple gives the coordinates of a hexagon in an analogous three-dimensional coordinate system for each of the two overlaid hexagon tilings. The number of tiles that are ${\displaystyle i}$ steps away from any given tile, for ${\displaystyle i=0,1,2,\dots }$, is given by the coordination sequence $${\displaystyle 1,5,11,16,21,27,32,37,\dots }$$ in which, after the first three terms, each term differs by 16 from the term three steps back in the sequence. One can also define analogous coordination sequences for the vertices of the tiling instead of for its tiles, but because there are two types of vertices (of degree three and degree four) there are two different coordination sequences arising in this way. The degree-four sequence is the same as for the square grid.