AMERICAN-STYLE OPTIONS BASICS
Plain English
Picture two movie tickets that expire in a month. One (American-style) lets you walk in on any day before it expires. The other (European-style) only works for the very last showing. If you sold the flexible ticket, you have to keep a seat ready every single day — you never know when they'll show up. In options, that seat is either the stock you might have to deliver (calls) or the cash you might have to pay (puts). That constant readiness is the heart of early-exercise risk.
Going deeper
An American-style option grants the holder the right, but not the obligation, to buy (call) or sell (put) the underlying asset at the strike price K on any trading day t ≤ T, where T is expiration. Because of the additional exercise opportunities, an American option is always worth at least as much as an equivalent European option. Key mechanics: the exercise window is open from listing until 4:30 PM ET on expiration Friday. Early exercise of calls is rational when the upcoming dividend exceeds remaining extrinsic value. For puts, early exercise becomes attractive when interest saved from receiving the strike price in cash exceeds remaining extrinsic. The OCC randomly assigns exercise notices to clearing firms, which then pass assignment to a customer account holding the short contract — assignment is final and immediate.
Examples
Dividend-Driven Call Assignment
MSFT trades at $340 and goes ex-dividend tomorrow for $0.75. You are short one MSFT 300-strike call expiring in two weeks. Intrinsic value = $40, option price = $40.50 (extrinsic = $0.50). Because $0.50 < $0.75, the holder gains $0.25 by exercising tonight to capture the dividend. You may be assigned, obliged to deliver 100 shares at $300, and you will not receive the dividend.
Why You Should Sell Rather Than Exercise a Put
SPY is at $430. You own a SPY 460-strike put expiring in five weeks, quoted at $30.25 ($30 intrinsic + $0.25 extrinsic). Exercising nets $30 but sacrifices the $0.25 extrinsic. Selling the put in the market captures the full $30.25 — always compare sale price vs. exercise proceeds before acting.
Assignment Risk Inside a Vertical Spread
You hold 50 AAPL 180/190 call spreads expiring in three days. AAPL closes at $189.90. The short 180 call is $9.90 ITM with just $0.05 extrinsic. If assigned on all 50 short calls, you open the next session short 5,000 AAPL shares at $180 while still long 50 calls at $190 — a −$450,000 stock position that must be managed immediately.