Question: The positions of the feet in ballet is a fundamental part of classical ballet technique that defines standard placements of feet on the floor, two additional positions, known as the sixth and seventh positions, were codified in the 1930s by Serge Lifar, who was a French ballet dancer and choreographer of Ukrainian origin, famous as one of the greatest male ballet dancers of which century?
Context: The Royal Ballet School is one of the world's greatest centres of classical ballet training.  The School's aim is to train and educate outstanding classical ballet dancers for The Royal Ballet (based at the Royal Opera House in London) and Birmingham Royal Ballet.  Admission to the School is based purely on talent and potential, regardless of academic ability or personal circumstances, and 90% of current students rely on financial support to attend the School.  The school is based over two sites, White Lodge, Richmond Park (for students aged 11–16) and Covent Garden (for students from 16–19 years old) based in purpose-built studios on Floral Street, adjacent to the Royal Opera House.  The Royal Ballet School has, for generations, produced dancers and choreographers of international renown, including Dame Margot Fonteyn, Dame Beryl Grey, Sir Kenneth MacMillan, Darcey Bussell, Alessandra Ferri and Viviana Durante, as well as current Director of The Royal Ballet Kevin O'Hare.  Graduates of the school have also achieved employment in musical theatre, contemporary and jazz dance, television and film. Igor Youskevitch (Russian: Игорь Юшкевич , Ukrainian: Ігор Юшкевич ) (13 March 1912 – 13 June 1994) was a ballet dancer and a choreographer of Russian-Ukrainian origin, famous as one of the greatest male ballet dancers of the 20th century, as a master of the classic style, e.g., in "Afternoon of a Faun", and as a dance partner to Alicia Alonso. Pointe technique is the part of classical ballet technique that concerns "pointe work", in which a ballet dancer supports all body weight on the tips of fully extended feet within pointe shoes.  A dancer is said to be "en pointe" when the dancer's body is supported in this manner, and a fully extended vertical foot is said to be "en pointe" when touching the floor, even when not bearing weight.  Pointe work is performed while wearing pointe shoes, which employ structural reinforcing to distribute the dancer's weight load throughout the foot, thus reducing the load on the toes enough to enable the dancer to support all body weight on fully vertical feet. Dame Alicia Markova DBE (1 December 1910 – 2 December 2004) was an English ballerina and a choreographer, director and teacher of classical ballet.  Most noted for her career with Sergei Diaghilev's Ballets Russes and touring internationally, she was widely considered to be one of the greatest classical ballet dancers of the twentieth century.  She was the first British dancer to become the principal dancer of a ballet company and, with Dame Margot Fonteyn, is one of only two English dancers to be recognised as a prima ballerina assoluta.  Markova was a founder dancer of the Rambert Dance Company, The Royal Ballet and American Ballet Theatre, and was co-founder and director of the English National Ballet. The positions of the feet in ballet is a fundamental part of classical ballet technique that defines standard placements of feet on the floor.  There are five basic positions in modern-day classical ballet, known as the first through fifth positions.  In 1725, dancing master Pierre Rameau credited the codification of these five positions to choreographer Pierre Beauchamp.  Two additional positions, known as the sixth and seventh positions, were codified by Serge Lifar in the 1930s while serving as Ballet Master at the Paris Opéra Ballet, though their use is limited to Lifar's choreographies.  The sixth and seventh positions were not Lifar's inventions, but revivals of positions that already existed in the eighteenth century, when there were ten positions of the feet in classical ballet. Serge Lifar (, "Serhіy Mуkhailovуch Lуfar"; Russian: Серге́й Миха́йлович Лифа́рь , "Sergey Mikhaylovich Lifar") (15 April [O.S. 2 April] 1905 , Kyiv, Ukraine)15 December 1986, Lausanne, Switzerland) was a French ballet dancer and choreographer of Ukrainian origin, famous as one of the greatest male ballet dancers of the 20th century.  Not only a dancer, Lifar was also a choreographer, director, writer, theoretician about dance, and collector. A ballet dancer (Italian: ballerina ] fem., ballerino ] masc.)  is a person who practices the art of classical ballet.  Both females and males can practice ballet; however, dancers have hierarchy and strict gender roles.  They rely on years of extensive training and proper technique to become a part of professional companies.  Ballet dancers are at a high risk of injury due to the demanding technique of ballet. Contemporary ballet is a genre of dance that incorporates elements of classical ballet and modern dance.  It employs classical ballet technique and in many cases classical pointe technique as well, but allows greater range of movement of the upper body and is not constrained to the rigorously defined body lines and forms found in traditional, classical ballet.  Many of its attributes come from the ideas and innovations of 20th century modern dance, including floor work and turn-in of the legs. Milorad Mišković, also Milorad Miskovitch (Serbian Cyrillic: Милорад Мишковић, ] ) (born March 26, 1928 in Valjevo, Kingdom of Serbs, Croats and Slovenes, died June 21, 2013 in Nice, France) was a Serbian ballet dancer and choreographer.  His emigration to France in 1947 made him a "persona non grata" in Yugoslavia, but has also made possible for him to show the world his class, that has later earned him the title of one of the greatest ballet dancers in the world in the 1950s.  His greatest successes include the roles of Prometheus, Don Juan, Tristan, Hamlet, and Orestês.  He has worked with some of the most famous artists of his time, such as choreographer Serge Lifar, opera singer Maria Callas, as well as the world's most prominent ballerinas of his time, such as Zizi Jeanmaire, Yvette Chauviré, Margot Fonteyn and Alicia Markova.  In 1956, he founded his own company, which continued to tour for ten years.  In 1966, Mišković performed in Yugoslavia for the first time after his emigration. Vakhtang Mikheilis dze Chabukiani (Georgian: ვახტანგ ჭაბუკიანი ) (February 27, 1910 – April 6, 1992) was a Georgian ballet dancer, choreographer and teacher highly regarded in his native country as well as abroad.  He is considered to be one of the most influential male ballet dancers in history, and is noted for creating the majority of the choreography of the male variations which comprise the classical ballet repertory.  This includes such famous ballets as "Le Corsaire" and "La Bayadère".
Answer: 20th