iron pillar

In the game of Go (Wei Qi/Badouk), "iron pillar" refers to two stones of the same color placed in directly adjacent points, as shown below:

...
.o.
.o.
...

Of course, it could be formed horizontally as well. The iron pillar is a very strong shape, but has no flexibility. The two stones are connected, so they must live or die together; after forming an iron pillar, you lose the choice of sacrificing one of the stones without also losing the other. Therefore, it should only be played in areas where strength is important, i.e. if there are stones of the opponent's in the immediate vicinity.

It may seem trivial and a waste of time to have a special name for something as simple as two adjacent stones, but the iron pillar is often used as part of larger, more complicated shapes. One commonly occuring one that doesn't have its own special name is ikken tobi from the iron pillar, as shown here:

......
..oo..
....a.
.cbo..
......

It is more flexible than the iron pillar, since either side can be sacrificed, but it is also effectively connected if you want it to be, since the opponent cannot cut unless he plays at the point marked a first, in which case you can simply form a bamboo joint at b or a table shape at c (see bamboo joint and table shape nodes for explanations of these shapes... it's worth noting that a bamboo joint is just two parallel iron pillars with an empty row between them).

A mysterious iron pillar stands in the courtyard of the old Quwwat Ul Islam Masjid in New Delhi, India, near the Qutb Minar. The pillar predates the mosque, and was apparently part of a Hindu temple complex that previously occupied the site; a six-line Pali inscription indicates that it was initially erected outside a Vishnu temple, and was raised in memory of Gupta king Chandragupta Vikramaditya.

A local legend states that in later times, the Iron Pillar was used to determine who could become an archer - if a young man could stand with his back to the pillar and reach his arms all the way around so that his hands touched, he would qualify. Today tourists visiting the Masjid are quite fond of attempting this feat (it's actually rather difficult).

But the most mysterious aspect of the Iron Pillar is its material composition. The pillar is definitely made of iron, but in almost 2000 years it it has never corroded or rusted the slightest bit. There has been a lot of debate as to why this is, and numerous studies have been conducted on the pillar to ascertain its exact composition, with inconclusive results. Did the Guptas have a secret formula for making super-strong iron, or was it just by strange happenstance that the iron composition of this particular pillar is so durable?