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This pattern of ten dots arranged in four rows to form the outline of an equilateral triangle, is known as a tetractys. Some call it a decad, which seems somehow less appropriate to me.
The word Tetractys comes from the Greek root Tetra- (four rows), whereas the word decad — as in decade — means ten.
To some, it's a magical symbol. To others, it is the basis of a special kind of poem. Mathematicians see it as a representation of the triangular number, 10, while (some) musicians see it as a representation of the pentatonic scale. Yet others see it every time they go bowling.
A tetractys poem has a special structure. You might call it the Brits' answer to a haiku:
First line - 1 syllable
Second line - 2 syllables
Third line - 3 syllables
Fourth line - 4 syllables
Fifth line - 10 syllables
On Scafell Pike
Stone
upon
huge stone piled -
a devil's cairn
Antlike I clamber towards a grey sky.
by Ray Stebbing