I know I'm a little late here, but I thought it might be worth mentioning that Wikipedia has a great list of base systems, which goes all the way up to 16 (Hexadecimal, of course) without holes, and then on to 85 (Pentaoxagesimal). Here's a quick reproduction of part of it:

1. unary (not actually on the main list, but listed farther down as being used in tally marks)
2. binary
3. ternary
4. quarternary
5. quinary
6. senary
7. septenary (used in weeks)
8. octal
9. nonary
10. decimal (everybody's favorite!)
11. undecimal
12. duodecimal (used in hours, months)
13. tridecimal
14. pentadecimal
15. pentadecimal
16. hexadecimal (Base16 encoding)

18 is octodecimal

20 is vigesimal

It's interesting to note that even our method of naming these systems reflects our attachment to the decimal system, as we begin to add prefixes after decimal. Also, if you want to form a higher number, it appears that you can use the following formula:

prefix for 2nd digit + prefix for 1st digit + gesimal

So, 27 is septemvigesimal. I invented this formula in answer to this question, but it appears to fit every case on the list.

Wikipedia also lists −2 as negabinary and −3 as negaternary. Theoretically, you can add the nega- prefix to anything, but I have no idea what you would use it for.

I know I'm a little late here, but I thought it might be worth mentioning that Wikipedia has a great list of base systems, which goes all the way up to 16 (Hexadecimal, of course) without holes, and then on to 85 (Pentaoxagesimal). Here's a quick reproduction of part of it:

1. unary (not actually on the main list, but listed farther down as being used in tally marks)
2. binary
3. ternary
4. quarternary
5. quinary
6. senary
7. septenary (used in weeks)
8. octal
9. nonary
10. decimal (everybody's favorite!)
11. undecimal
12. duodecimal (used in hours, months)
13. tridecimal
14. tetradecimal  
15. pentadecimal
16. hexadecimal (Base16 encoding)

18 is octodecimal

20 is vigesimal

It's interesting to note that even our method of naming these systems reflects our attachment to the decimal system, as we begin to add prefixes after decimal. Also, if you want to form a higher number, it appears that you can use the following formula:

prefix for 2nd digit + prefix for 1st digit + gesimal

So, 27 is septemvigesimal. I invented this formula in answer to this question, but it appears to fit every case on the list.

Wikipedia also lists −2 as negabinary and −3 as negaternary. Theoretically, you can add the nega- prefix to anything, but I have no idea what you would use it for.

I know I'm a little late here, but I thought it might be worth mentioning that Wikipedia has a great list of base systems, which goes all the way up to 16 (Hexadecimal, of course) without holes, and then on to 85 (Pentaoxagesimal). Here's a quick reproduction of part of it:

1. unary (not actually on the main list, but listed farther down as being used in tally marks)
2. binary
3. ternary
4. quarternary
5. quinary
6. senary
7. septenary (used in weeks)
8. octal
9. nonary
10. decimal (everybody's favorite!)
11. undecimal
12. duodecimal (used in hours, months)
13. tridecimal
14. pentadecimal
15. pentadecimal
16. hexadecimal (Base16 encoding)

18 is octodecimal

20 is vigesimal

It's interesting to note that even our method of naming these systems reflects our attachment to the decimal system, as we begin to add prefixes after decimal. Also, if you want to form a higher number, it appears that you can use the following formula:

prefix for 2nd digit + prefix for 1st digit + 'gesimal'

So, 27 is septemvigesimal. I invented this formula in answer to this question, but it appears to fit every case on the list.

Wikipedia also lists -2 as negabinary and -3 as negaternary. Theoretically, you can add the nega- prefix to anything, but I have no idea what you would use it for.

I know I'm a little late here, but I thought it might be worth mentioning that Wikipedia has a great list of base systems, which goes all the way up to 16 (Hexadecimal, of course) without holes, and then on to 85 (Pentaoxagesimal). Here's a quick reproduction of part of it:

1. unary (not actually on the main list, but listed farther down as being used in tally marks)
2. binary
3. ternary
4. quarternary
5. quinary
6. senary
7. septenary (used in weeks)
8. octal
9. nonary
10. decimal (everybody's favorite!)
11. undecimal
12. duodecimal (used in hours, months)
13. tridecimal
14. pentadecimal
15. pentadecimal
16. hexadecimal (Base16 encoding)

18 is octodecimal

20 is vigesimal

It's interesting to note that even our method of naming these systems reflects our attachment to the decimal system, as we begin to add prefixes after decimal. Also, if you want to form a higher number, it appears that you can use the following formula:

prefix for 2nd digit + prefix for 1st digit + gesimal

So, 27 is septemvigesimal. I invented this formula in answer to this question, but it appears to fit every case on the list.

Wikipedia also lists −2 as negabinary and −3 as negaternary. Theoretically, you can add the nega- prefix to anything, but I have no idea what you would use it for.