Senary

"Senary" refers to the base-6 numeral system.

All prime numbers besides 2 and 3 end in either 1 or 5.

 2,3,5,11,15,21,25,31,35,45,51,101,105,111,115,125,....

All known perfect numbers besides 6 itself end in 44.

Base 6 can be counted on hands by using each hand as a digit (0 to 5).

The senary number 3333 is equal to decimal 777.

Fractions

Six is the product of the first two prime numbers and is adjacent to the next two prime numbers. This makes many senary fractions simple.

 Decimal   Senary
   1/2     1/2   =  0.3
   1/3     1/3   =  0.2
   1/4     1/4   =  0.13
   1/5     1/5   =  0.1111 recurring
   1/6     1/10  =  0.1
   1/7     1/11  =  0.05050505 recurring
   1/8     1/12  =  0.043
   1/9     1/13  =  0.04
   1/10    1/14  =  0.03333 recurring

   1/12    1/20  =  0.03
   1/14    1/22  =  0.023232323 recurring
   1/15    1/23  =  0.022222222 recurring
   1/16    1/24  =  0.0213

See also: octal, decimal, duodecimal and hexadecimal.