<Tr> <Td> Week 6 </Td> <Td_colspan="2"> Italian Open (Rome, Italy) </Td> </Tr> <Tr> <Td> Week 7 </Td> <Td> Geneva Open (Geneva, Switzerland) Open de Nice Côte d'Azur (Nice, France) </Td> <Td> Internationaux de Strasbourg (Strasbourg, France) Nuremberg Cup (Nuremberg, Germany) </Td> </Tr> <Tr> <Td> Week 8 </Td> <Td_colspan="2"> French Open (Paris, France) </Td> </Tr> <Tr> <Td> Week 9 </Td> </Tr>

What are the lines on clay tennis courts made of