# 19

This number is a prime.

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Consider a square made of four rigid rods, hinged at the ends. How many rods of the same type must be added to make the square rigid in the plane? The answer is 19, which makes a total of 23 rods (also prime). [McCranie]

Not counting Panno's resignation (1. c4 resigns), Bobby Fischer won 19 consecutive games at the World Championship Interzonal tournament and Candidates matches in 1970-71. [McCranie]

An unistable polyhedron is stable on only one face. The simplest such polyhedron known requires 19 faces. Whether unistability is possible with fewer faces is an unsolved problem. [McCranie]

In chess positions with a king and pawn versus a king in which pawn promotion can be forced, at most 19 moves are required to promote the pawn (with best play by both sides). [McCranie]