Option Series

An option series is the complete set of identical option contracts, defined by three fixed parameters: the underlying asset, the expiration date, and the strike price. Every call with those coordinates belongs to one series; every put with those coordinates to another. All contracts within a series have identical rights, risks, and pricing.

What defines a series

A series is determined by exactly three dimensions:

The underlying. IBM common stock, the S&P 500 index, crude oil, EUR/USD—the asset you can buy or sell via the option.

The expiration date. Typically the third Friday of the month for equity options; quarterly for index options; or monthly and weekly variants on many exchanges. All contracts expiring on the same calendar date belong to the same series.

The strike price. The fixed price at which the option buyer can exercise. A £50 strike, a £55 strike, and a £45 strike are three different series.

Once all three parameters align, every contract is fungible. One call option with underlying XYZ, expiry in March, strike £50 is interchangeable with every other such call. You can buy 10 contracts and sell 5 without naming which specific certificates you’re trading—they’re all identical.

Series vs. class vs. the entire option chain

The terminology is nested:

Class. All options on a single underlying (e.g., “Apple options” or “crude oil options”).

Series. The slice defined by expiry + strike + right (e.g., “Apple March £150 calls”).

The option chain. The full menu of available series on a given underlying. On a major stock, this might run to dozens of expirations and dozens of strike prices per expiry, yielding hundreds of series.

Why series structure matters

Series structure enables price transparency and price discovery. All participants—market makers, floor traders, algorithm engines—trade contracts within the same series at the same bid-ask spread, creating a unified market price. A trader checking the bid on Apple March £150 calls gets the live consensus of all open interest in that exact series.

Fungibility also powers margin and leverage. You can buy 100 shares and sell a covered call in the same series; the broker knows the series perfectly offsets your stock risk if the call is exercised. Exchanges and clearing houses can net positions across the same series, reducing counterparty exposure.

Open interest per series

Open interest is the total number of contracts outstanding in a series at any given moment. A heavily traded series (e.g., Apple near-month at-the-money options) might have millions of contracts open. A far-out expiry at an exotic strike might have zero.

Liquidity pools by series. If you want to sell 1,000 contracts, the series with the highest open interest offers the tightest spread and fastest execution. A thinly-traded series might have a spread of 25 cents; the popular one might be 1 cent.

Standard vs. non-standard series

U.S. equity options follow standardized series: expirations on the third Friday of named months, monthly and now weekly expirations, and strikes set at fixed intervals (typically £1 or £2.50 apart for stocks trading under £200). This standardization reduces confusion and maximizes liquidity.

Over-the-counter options or bespoke structures might skip standard series altogether. A derivatives dealer and a client might negotiate a custom expiry (any date they choose) and custom strike (any price), creating an entirely non-standard series of one. That one-off contract is still a series—just with no other members.

Expiration cascades

On any given day, multiple series of the same underlying expire in sequence: front-month (nearest), then second month, then quarterly months out to a year or more. As the front-month series nears expiry and time decay accelerates, traders often roll positions—closing the near-month series and opening the same strike in the next series out. This practice keeps liquidity fresh in the front-month series and smooths the transition from one series to the next.

Series and contract specifications

The exchange (or OTC counterparty) defines each series’s fine print: multiplier (how many shares one option contract represents, typically 100 for equity options), settlement method (cash or physical delivery), and whether the option is American-style (exercisable anytime) or European-style (exercisable only at expiry). These specifications apply uniformly to all contracts in the series.

See also