*A photo of the MO-Labs “Clebsch diagonal surface with two planes” model shot in the MO-Labs office, on a table made from a beautiful 100 year-old piece of wood found in a barn. To purchase the model used in the photo and others, follow the links to our producer “shapeways” provided below.*

Clebsch’s diagonal surface contains 27 real straight lines. The fact that any smooth cubic contains 27 straight lines (possible imaginary) was known since 1849. The British lawer/mathematician Arthur Cayley first found that there may only be finitely many lines. When he wrote a letter about this to his friend George Salmon, he almost immediately received the answer saying that their number is exactly 27.

One of the simplest ways to count this number starts with one line, and considers all planes through this line. It turns out that in five special positions the plane cuts the surface in two more lines besides the fixed one. This is the main ingredient of the argument.

Our model shows two such planes: One passing through a line and intersecting the surface in a smooth conic section (a hyperbola, in fact), and another one passing through a line and intersecting the surface in two more lines (thus in three lines in total). Such planes are called triply tangent planes, or tritangent planes for short, because they are indeed tangent to the surface in the three intersection points of the three lines.

This sculpture also appears in our article “Straight lines on models of curved surfaces” (to appear in 2017 in Mathematical Intelligencer). For all our math sculptures mentioned in this article see the corresponding category article “Straight lines on models of curved surfaces”.

#### MO-Labs sculptures used: Clebsch’s diagonal surface with two planes

To purchase this exhibit size (20.5 cm / 8.1 in) version or a slightly smaller version (17.5 cm / 6.9 in) of Clebsch’s diagonal surface with two planes shown in the photo above, just use the “buy now”-link provided below. You will be sent to our MO-Labs shop on the “shapeways” website, so that you can let “shapeways” produce the object for you and send it to you in really short time, where ever you are located in the world.

#### Related Sculptures

We have designed a large variety of other Clebsch diagonal surface sculptures, in one color, or with differently colored lines, or even together with two related planes. We provide versions in different sizes and at very different prices, from tiny and cheap to large and more valuable. Here are some of our posts related to the Clebsch diagonal surface:

#### The Clebsch diagonal surface: 27 lines only

The sculpture we present here is a 3D-printed modern object consisting of the 27 lines only, and a thin part of the surface as a border.

#### The Clebsch diagonal surface with colored lines

This version of Clebsch's famous diagonal surface model features colored lines. One intersects the surfaces in a line and a hyperbola, the another one in three lines.

#### The Clebsch diagonal surface with two planes

This version of Clebsch's famous diagonal surface model features colored lines and two additional planes. One intersects the surfaces in a line and a hyperbola, the another one in three lines.

#### Clebsch and Klein in the family room

It was back in the 1872 Göttingen, Germany, at a meeting of the scientific society. Alfred Clebsch and Felix Klein each presented a model of a cubic surface. Our modern versions of these historical - nowadays quite famous - sculptures are the main figures in our photo.

#### The Clebsch diagonal surface in the family room

Our modern version of Clebsch's historical - nowadays quite famous - diagonal surface model is the main figure in our photo from the series "math sculptures in context". It is the pure white version with the 27 straight lines.

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