Allowed characters for NCName

Hi,

In my xml docs I have an atrribute of type xs:NCName.

When validating the xml against a schema the Turkish lower case i Character: ı is not allowed in the attribute.

From the XML Schema recommendation here http://www.w3.org/TR/xmlschema-2/#NCName
i know that:

NCName          ::=     (Letter | '_') (NCNameChar)*
NCNameChar     ::=     Letter | Digit | '.' | '-' | '_' | CombiningChar | Extender

My questions are:
1. Is Letter = any letter in the English Alphabet (of any case)?
2. What are the CombiningChar(s)?
3. What are the Extender(s)?
4 Obviously Digit = numbers (0-9).