What makes Web service 'interoperable'?

Well we know that XML, SOPA, WSDL and UDDI are the building blocks of WEb services.

I would like to know which is the main thing which makes web services interoperable.

I feel, XML is the main thing because,
1) all other things are based on it
2) through schemas it provide some standards.

Please correct me if i am wrong.
Please let em know your comments also.

Interoperability is ensured by basing WS on standards like HTTP and XML which have well-defined behavior when it comes to platform differences (like character encodings, line endings and byte order).

Once you have figured out how to transport XML over HTTP without having to worry about platform differences, everything else (SOAP, WSDL, WS-Security, etc.) are just XML applications on top of that.
[ July 23, 2008: Message edited by: Ulf Dittmer ]