I was actually thinking about this same question earlier. There are dialects of Lisp and dialects of BASIC, but most other languages are not considered dialects of anything similar. How different do two programming languages have to be before they're no longer considered dialects?

It seems like Scheme and Common Lisp, for instance, have more differences than Java and C# have from each other. Why wouldn't C# be considered a Java dialect? Visual Basic is very different from BASICA, yet they are both considered BASIC dialects. Why shouldn't Pascal and Ada be considered ALGOL dialects?

The line-drawing process seems completely arbitrary.