The Purchase Order Schema, po.xsd 
            <xsd:schema xmlns:xsd="http://www.w3.org/2000/08/XMLSchema">

             <xsd:annotation>
              <xsd:documentation>
               Purchase order schema for Example.com.
               Copyright 2000 Example.com. All rights reserved.
              </xsd:documentation>
             </xsd:annotation>

             <xsd:element name="purchaseOrder" type="PurchaseOrderType"/>

             <xsd:element name="comment" type="xsd:string"/>

             <xsd:complexType name="PurchaseOrderType">
              <xsd:sequence>
               <xsd:element name="shipTo" type="USAddress"/>
               <xsd:element name="billTo" type="USAddress"/>
               <xsd:element ref="comment" minOccurs="0"/>
               <xsd:element name="items"  type="Items"/>
              </xsd:sequence>
              <xsd:attribute name="orderDate" type="xsd:date"/>
             </xsd:complexType>

             <xsd:complexType name="USAddress">
              <xsd:sequence>
               <xsd:element name="name"   type="xsd:string"/>
               <xsd:element name="street" type="xsd:string"/>
               <xsd:element name="city"   type="xsd:string"/>
               <xsd:element name="state"  type="xsd:string"/>
               <xsd:element name="zip"    type="xsd:decimal"/>
              </xsd:sequence>
              <xsd:attribute name="country" type="xsd:NMTOKEN"
                 use="fixed" value="US"/>
             </xsd:complexType>

             <xsd:complexType name="Items">
              <xsd:sequence>
               <xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
                <xsd:complexType>
                 <xsd:sequence>
                  <xsd:element name="productName" type="xsd:string"/>
                  <xsd:element name="quantity">
                   <xsd:simpleType>
                    <xsd:restriction base="xsd:positiveInteger">
                     <xsd:maxExclusive value="100"/>
                    </xsd:restriction>
                   </xsd:simpleType>
                  </xsd:element>
                  <xsd:element name="USPrice"    type="xsd:decimal"/>
                  <xsd:element ref="comment"   minOccurs="0"/>
                  <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
                 </xsd:sequence>
                 <xsd:attribute name="partNum" type="SKU"/>
                </xsd:complexType>
               </xsd:element>
              </xsd:sequence>
             </xsd:complexType>

             <!-- Stock Keeping Unit, a code for identifying products -->
             <xsd:simpleType name="SKU">
              <xsd:restriction base="xsd:string">
               <xsd:pattern value="\d{3}-[A-Z]{2}"/>
              </xsd:restriction>
             </xsd:simpleType>

            </xsd:schema>