Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Outcome | Year | Championship | Surface | Partner | Opponents | Score |
| --- | --- | --- | --- | --- | --- | --- |
| Winner | 1960.0 | Wimbledon Championships | Grass | Rafael Osuna | Mike Davies Bobby Wilson | 7–5, 6–3, 10–8 |
| Winner | 1961.0 | US Championships | Grass | Chuck McKinley | Rafael Osuna Antonio Palafox | 6–3, 6–4, 2–6, 13–11 |
| Runner-Up | 1962.0 | US Championships | Grass | Chuck McKinley | Rafael Osuna Antonio Palafox | 6–4, 10–12, 1–6, 9–7, 6–3 |
| Winner | 1963.0 | US Championships (2) | Grass | Chuck McKinley | Rafael Osuna Antonio Palafox | 9–7, 4–6, 5–7, 6–3, 11–9 |
| Winner | 1964.0 | US Championships (3) | Grass | Chuck McKinley | Mike Sangster Graham Stilwell | 6–3, 6–2, 6–4 |
| Winner | 1966.0 | French Championships | Clay | Clark Graebner | Ilie Năstase Ion Ţiriac | 6–3, 6–3, 6–0 |
| Runner-Up | 1966.0 | US Championships (2) | Grass | Clark Graebner | Roy Emerson Fred Stolle | 6–4, 6–4, 6–4 |
| Runner-Up | 1969.0 | US Open (3) | Grass | Clark Graebner | Ken Rosewall Fred Stolle | 2–6, 7–5, 13–11, 6–3 |
Question:
What is the surface of the French Championships?
Answer:
Clay