Strobogrammatic number Strobogrammatic number A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69, 96, 1001). A strobogrammatic prime is a strobogrammatic number that is also a prime number, i.e., a number that is only divisible by one and itself (e.g., 11). It is a type of ambigram, words and numbers that retain their meaning when viewed from a different perspective, such as palindromes. When written using standard characters (ASCII), the numbers, 0, 1, 8 are
Prime (symbol) sequences (e.g., "L" means "turn the left face 90 degrees clockwise", whereas "L′" means "turn the left face 90 degrees counter-clockwise"). The prime symbol is used in combination with lower case letters in the Helmholtz pitch notation system to distinguish notes in different octaves from middle C upwards. Thus c represents the C below middle C, c′ represents middle C, c″ represents the C in the octave above middle C, and c‴ the C in the octave two octaves above middle C. A combination of upper case letters and sub-prime symbols is used to represent notes in lower octaves. Thus