Investopedia explains 'Form T'
Trading in extended hours enables investors to react quickly to events that typically occur outside regular market hours, such as earnings reports. However, liquidity may be constrained during such Form T trading, resulting in wide bid-ask spreads. Form T trading is especially suited for overseas investors, since they may conduct the bulk of their U.S. trading when their markets are open but U.S. markets are closed.

The growing popularity of electronic communication networks means that Form T trading is bound to continue increasing.

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