Regular trading session Extended-hours sessions
Orders can be placed at any time and will beexecuted only from 9:30 a.m. to 4:00 p.m. ET.
Orders can be placed for the following sessions: 

For orders placed on Schwab.com or Schwab Mobile: 

7:00 a.m to 8:00 p.m ET with five-minute closures before and after regular market hours. 

For orders placed on thinkorswim platforms: 

7:00 a.m. to 8:00 p.m. ET with five-minute closures before and after regular market hours.

Overnight trading session (EXTO) orders are 24-hour continuous orders that expire at 8 p.m. ET every market day. For example, an EXTO order placed at 2 a.m. ET Monday morning would be active immediately and remain active from then until 8 p.m. ET Monday night. A trade placed at 9 p.m. ET Monday night would be active immediately and remain active until 8 p.m. ET Tuesday night. The overnight trading session is limited to a select number of securities. 
Trading occurs on exchanges like the New YorkStock Exchange (NYSE) and Nasdaq, and through a variety of venues, includingmarket makers and other market centers.  Similar to regular market sessions, trading occurs on exchanges like the New York Stock Exchange and Nasdaq and through a variety of venues, including market makers and other market centers. 
Many order types and restrictions are accepted,including: market, limit, stop-limit, all-or-none, etc. Only limit orders are accepted.
All order sizes are accepted.

For pre-market and after-market trading sessions on Schwab.com, there is no maximum quantity on a single order. 

 

For an overnight trading session, 50,000 shares, or a notional value of $10 million dollars, is the maximum quantity on a single order. 
Many security types are available, including:stocks, options, bonds, mutual funds, etc. Most listed and Nasdaq securities are available in the extended hours session.
Different timing and order choicesare available, including: Day, GTC, IOC and FOK.

Pre-market, after-hours, and during the overnight session, order types are good only during the session for which they were placed. Seamless orders that participate in pre-market, regular market, and after-hours trading are available to be placed GTC. The overnight trading session is limited to a select number of securities. 

 

Note that GTC_EXT must be chosen to place overnight trades on thinkorswim trading platforms.
In general, higher trading activity means moreliquidity and a greater likelihood of order execution. Lower trading activity may lower the likelihood of an order being executed, as well as wider spreads and greater price fluctuations.
The quotes you receive are consolidated and represent the best available prices across all trading venues. exchanges, Market makers and specialists work to ensure customers get the best available buy or sell prices.  Quotes and fills aren't consolidated and represent the current prices available through the market. As a participant in the extended-hours trading network, the market may also offer access to prices available in other participating markets but not necessarily all venues open for extended-hours trading. Best execution isn't guaranteed.