include

Hello, I am looking for help about <xs:include ... />

let's say my schema 'a.xsd' includes one other schemas :  'b.xsd'  
The thing is I want to declare/use some similars elements in 'a.xsd' and 'b.xsd' . 
These common elements are declared in 'c.xsd' . 
Do I have to include 'c.xsd' in 'a.xsd' and in 'b.xsd'
or in 'b.xsd' only? 
or in 'a.xsd' only? 

I know that with the dtd's, it worked with the entities. I did "include" dtd's and I didn't need to declare two times my elements but I don't know with schemas? 

thank you for your help...

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Florent Claude 
InOvo 
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