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According to italian theatre historians, such as Roberto Tessari, Ferdinando Taviani, and Luciano Pinto, Commedia dell’arte, which literally translated as improvised comedy, developed as a response to the political and economic crisis of the 16th century in Italy and, as a consequence, became the first entirely professional form of theatre .
It is believed that the use of mask in commedia dell’atre originated in 1570 with Andrea Calmo, the actor and author who was inspired by the venetian carnivals of the time. She created Pantaloon who is the one of the most famous characters of commedia dell’arte today. She went on to create the spanish captain ‘Il Capitano’. Although her characters were initially unmasked, Andrea wished that the spirit of the Italian carnival ( which celebrates the epiphany  ) would be represented in her plays and therefore she introduced the masks and thus created the commedia dell’arte that we know today. 
The carnival of venice. 
Records of commedia dell’arte performances date back as early as 1551, where they were acted by professionals outside on temporary stages with costumes and masks. But as it was so popular with the nobles, many performances were also done indoors at court for the amusement of nobles. 
After it became famous with the nobles the troupes began to move to france where many play writes such as Moliere were inspired to move their written works towards comedy.
Commedia dell’arte started to become popular outside Italy in the beginning of the seventeenth century. As the ‘tipo fisso’ (fixed stereotypes) soon became satiric references to the Italian parliament and to the typical Italian person it became one of the most popular comic theatre genres in the 17th and 18th century. 
There are several reasons for the use of masks in commedia dell’atre. Primarily, as this type of theatre has fixed characters but is essentially improvised, the masks are used so that the audience was able to identify the characters. As commedia dell’arte was performed by different travelling companies, this meant that they would have different costumes, yet the masks would retain all the information needed to recognise a specific character. 
Also as these performances were mainly on the streets or by the road, there would not really be a stage nor seating for the audience this made it difficult for everyone to see everything from the same angle or height. With the masks, all the audience could observe the emotions or actions that were being portrayed.
Commedia dell’arte masks were traditionally made of leather and only covered half of the actor’s face. They were like this so that the actor was able to project his voice and made certain noises that he could not do with a full mask. The masks were made to fit the face of a given actor because he or she would generally act out the same character for his/her entire career. Commedia dell’arte masks tend to have large, broad feature which are distinct to each character. For example on the character Zanni, the nose is very large. It is said that the larger the nose is, the stupider the Zanni represented tends to be.
Zanni  Zanni 
As I have said before, commedia dell’arte characters were based on ‘tipo fisso’ (a certain stereotype). Some of them have changed there name or personality over time for example Zanni, who is characterised as short of wit and always without money. Zanni has several masks that represent him which have changed over time with the fashion.
Which all mean different things for example the Brighella mask which represents that poor and starving and hopeless side of Zanni and the Pucinella mask which represents the side which is also poor but although he is stupid he is able to outwit many. Many of the characters had a stereotypical opposite for example the stereotypical opposite of Zanni is represented with the well known character Pantalone (Pantaloon) who is typically arrogant and rich and who is always dealing with people who are trying to take his money away from him.
Although most of the commedia dell’arte characters wore masks, there were few that were never masked. For example Isabella. Who was most famously acted by the famous actress Isabella Andreini. Also the lovers who were mostly the daughters and sons of the old and rich such as Pantalone or The Captain did not wear masks. This is because they were just there to bring the satiric comedy of romance and love to the performance. They were there to make the performance more light. And therefore did not have a specific characteristic apart from being completely in love with their character. 
Commedia dell’arte masks were useful in the sense that although the the costumes and stage changed in fashion over time, The masks stayed basically the same. In this way the audience could appreciate and recognise the character and still enjoy new ways of the typical story being presented.
In many schools such as Jaques Lecoq’s international school of theatre in paris, commedia dell’arte masks are used as a learning instrument for aspiring actors. Jaques Lecoq, was born in 1921 started off as a sports teacher and then moved on to acting with his partner, Gabreielle Cousin. He moved to Italy for eight years in 1948 where he discovered Commedia dell’arte. Together with the sculptor, Amleto Sartori, they developed the neutral mask. When Lecoq moved back to Paris he devoted the remainder of his life to teaching at his school.
Jaques Lecoq 
His school offers several courses that work on understanding and working with the body through the use of masks. His philosophy was that in order to become a good actor, you needed to use the Neutral masks to discover your body, movement, balance and space. He also believed that the typical half masks of commedia dell’arte, played an important role in improvisation and to work on character types therefore he integrated both into his courses .
Words 1000 (not including bibliography, notes under images nor titles )
 – http://en.wikipedia.org/wiki/Commedia_dell’arte
 – http://www.religionfacts.com/christianity/holidays/mardi_gras.htm
 – http://italian.about.com/library/weekly/aa110800a.htm
 – http://www.usq.edu.au/artsworx/schoolresources/androclesandthelion/commedia#Mask
 – http://wiki.answers.com/Q/What_are_masks_used_for
 – http://italian.about.com/library/weekly/aa110800a.htm
 – http://www.ecole-jacqueslecoq.com
Bibliography of Images:
 – http://travel.smart-guide.net/venice-carnival
 – http://tombanwell.blogspot.com/2010/09/zanni-leather-mask.html
 – http://www.theater-masks.com/commedia-masks/commedia-mask-zanni-1
 – http://www.clg-mignet.ac-aix-marseille.fr/mignet/spip.php?article440
 – http://www.maghress.com/fr/marochebdo/31832