Question: When was the first tiering system in women's tennis introduced?
About the article "Tennis", section "Premier events".

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Women's tournament structure.
Premier events.
Premier events for women form the most prestigious level of events on the Women's Tennis Association Tour after the Grand Slam tournaments.
These events offer the largest rewards in terms of points and prize money.
Within the Premier category are Premier Mandatory, Premier 5, and Premier tournaments.
The Premier events were introduced in 2009 replacing the previous Tier I and II tournament categories.
Currently four tournaments are Premier Mandatory, five tournaments are Premier 5, and twelve tournaments are Premier.
The first tiering system in women's tennis was introduced in 1988.
At the time of its creation, only two tournaments, the Lipton International Players Championships in Florida and the German Open in Berlin, comprised the Tier I category.
The WTA Swiss Open was a women's professional tennis tournament played in a number of locations in Switzerland.
The event was held from 1899–1994, and was played on outdoor clay courts.
The tournament underwent a name change in 1986, when it was titled the European Open until its discontinuation.
It formed part of the Women's Tennis Association (WTA) Tour.
When the WTA introduced the tiering format to its circuit, the event gradually moved up, from being a Tier V in 1988–1989, a Tier IV from 1990–1992, and a Tier III for its remaining years.
Two Swiss players won the event: Manuela Maleeva (who formerly represented Bulgaria) won the singles in 1991, and Christiane Jolissaint, who won the doubles on three occasions: 1983, 1984, and 1988.
Maleeva holds the record, along with Chris Evert, for most singles wins; both players won the event three times, and Maleeva finished runner-up a further three occasions. 
Below are the complete results of the Women's Singles tennis competition during the 1984 Summer Olympics in Los Angeles, when tennis was re-introduced as a demonstration sport.
The tournament was for players aged 21 or under. 
This is a list of women's historical number one ranked tennis players, traditionally determined at year-end.
Notes: The Women's Tennis Association introduced computerized ranking system in November 1975, which is incorporated into this list.
The International Tennis Federation's year-end number 1 classification that was introduced in 1978 is no longer named as such, and has evolved into the ITF World Champion designation and award.  
The on-disk layout of VxFS is versioned and upgradeable while the file system is mounted.
This file system has gone through seven versions.
Version 2 added support for filesets, dynamic inode allocation and ACLs.
Layouts 1-3 stopped being supported in VxFS 4.0.
Version 4 added support for storage checkpoints and for Veritas Cluster File System.
Version 4 was released in VxFS 3.2.1.
Layout version 4 is no longer supported under VxFS 5.1 [http://seer.entsupport.symantec.com/docs/339064.htm].
Version 5 started support for file systems up to 32 terabytes () in size.
Individual files can be up to 2 terabytes in size.
Version 5 was introduced in VxFS 3.5 and is no longer supported under VxFS 5.1 [http://seer.entsupport.symantec.com/docs/339064.htm].
Version 6 added support for file systems and files up to 8 exabytes () in size.
Version 6 also introduced support for named streams/resource forks, for multiple underlying volumes, and for file change logs.
Version 6 was introduced in VxFS 4.0.
Version 7 extends support for multiple volumes to permit Dynamic Storage Tiering.
Dynamic Storage Tiering allows root users to move files among different volumes, allocate files to different volumes at file creation time based on policy, and independently recover volumes, without altering the namespace of the file system.
Version 7 was introduced in VxFS 5.0 It is assumed that transparent compression and transparent encryption will be addressed in Version 8 or Version 9.