In 1993 the Chicago Board Options Exchange developed FLexible EXchange Options (acronym -- "FLEX" options) to broaden institutional investor access to customized derivative products. These contracts are similar in structure to privately negotiated "over-the-counter" call and put options and warrants and are designed for initial transactions of at least $10,000,000 of notional principal. CBOE first introduced FLEX for the trading of index options; later, E-Flex options on individual equities were introduced. In simplest terms, FLEX options allow options users to select contract terms for exchange-listed options.
CBOE developed CFLEX in 2007 as the first Internet-based execution system for electronic trading of FLEX options. 
In 2009 the exchange eliminated blackout dates for FLEX options, making way for options trading on key dates.
In January of 2010, the CBOE said they had asked the Securities and Exchange Commission to broaden access to the FLEX options. The exchange expanded the range of dates the products could be traded and requested that the SEC reduce the size of the contracts to open them up to more kinds of traders. 
CBOE FLEX options volume grew from 967,000 contract in 2008 to a record 3.52 million contracts in 2009.
FLEX Options have several unique features as compared to OTC options:
- Options Clearing Corporation -- While an over-the-counter market currently exists for customized options, it cannot provide the issuance and clearing guarantee benefits of OCC.
- Contracts can be customized along four dimensions.
- Price discovery is present in a competitive auction market.
- Administrative convenience of listed options -- OCC marks prices on a daily basis for all outstanding FLEX positions. These marked prices are available to clearing members of OCC. In addition, on a request basis, Market Makers will value open FLEX series through the bid and offer process on the last business day of each month. No trading may take place during this valuation process.
Index FLEX Options also provide a secondary market to offset or alter positions and an independent daily marking of prices. Calls and puts on the S&P 100, S&P 500, Nasdaq 100, Russell 2000 and Dow Jones Industrial Average Indices provide the foundation for the FLEX system.
E-FLEX options (on individual equity options) were introduced after FLEX options, again to give users the ability to customize key contract terms, like exercise price, exercise style and expiration date. E-FLEX options also have no position limits restrictions.
Uses for E-Flex options:
- Of interest to holders of restricted stock, zero-cost collars can be established providing downside protection and limited upside gain at no net cost.
- Large stockholders can close their positions with lower market impact by selling customized calls that have been created as part of an exit strategy.
- Institutions can amplify protection for their stock holdings by buying customized puts, limiting downside risk and taking advantage of expanded position limits.
- Investors can establish a synthetic short stock position by simultaneously buying customized puts and selling customized calls, avoiding early stock recalls and dividend payment concerns.
- Investors can establish a synthetic long stock position -- at a lower cash outlay than an actual stock purchase -- by simultaneously selling customized puts and buying customized calls.
- Corporate stock repurchase programs can sell customized puts creating the potential to buy back stock at a predetermined price while generating additional income. 
In 2007, CBOE announced that it had entered into an agreement with Stockholm-based Cinnober Financial Technology for Cinnober to provide its trading technology to CBOE for the electronic trading of both equity and index FLEX options, thus providing a full-automated electronic system for these products.
- "FLEX® Options”. www.cboe.com/.
- CFLEX Options. CBOE.
- Chicago Board Options Exchange Ramping Up OTC Effort in 2010. The Wall Street Journal.
- "Flex Options”. www.cboe.com/.
- "Equity FLEX Options”. www.cboe.com/.
- CBOE announces new electronic system for trading flex options. Cinnober Financial Technology.