> ## Documentation Index
> Fetch the complete documentation index at: https://finance.dev/llms.txt
> Use this file to discover all available pages before exploring further.
# Single-Leg Option Order
> Buy or sell one option contract — a single call or put. Learn how the legs array works, position effect, and how to express to-open and to-close.
A **single-leg option order** buys or sells **one** option contract — one call or one put. You express it exactly like an equity order, **plus a `legs` array with one entry** describing the contract. The presence of that single leg is what tells the platform "this is an option order."
**The rule that drives everything:** the number of entries in `legs` decides the order shape — `0` = equity, `1` = single-leg option (this page), `2–4` = [multi-leg option](/api-reference/orders/multi-leg-option). See [Order Flow](/api-reference/orders/order-flow#how-the-order-type-is-decided).
***
## Anatomy of a single-leg option order
The **top-level** fields describe the order; the **leg** describes the contract.
| Level | Field | Notes |
| ----- | ---------------- | --------------------------------------------------------------------------------------------------------------- |
| Order | `symbol` | The **underlying** symbol (e.g. `AAPL`). |
| Order | `side` | `BUY` or `SELL` — the direction of the order. |
| Order | `type` | `MARKET`, `LIMIT`, `STOP`, or `STOP_LIMIT` (price fields per [those rules](/api-reference/orders/limit-order)). |
| Order | `qty` | Number of **contracts**. |
| Order | `timeInForce` | **`DAY` or `GTC` only** for options. |
| Order | `price` | Per-contract limit price, if `LIMIT`/`STOP_LIMIT`. |
| Leg | `securityType` | `OPT` for an option. |
| Leg | `side` | `BUY` or `SELL` (also accepts `B`/`S`). |
| Leg | `putCall` | `CALL` or `PUT` (also accepts `C`/`P`). |
| Leg | `strikePrice` | The strike, as a decimal string. |
| Leg | `maturity` | Expiration date, `YYYY-MM-DD`. |
| Leg | `positionEffect` | `OPEN` or `CLOSE` (also accepts `O`/`C`). |
| Leg | `ratioQty` | `"1"` for a single leg. |
| Leg | `symbol` | Optional; defaults to the order-level underlying. |
**`side` vs. `positionEffect`.** `side` is whether you're buying or selling the contract; `positionEffect` is whether that opens a new position or closes an existing one. A `BUY` + `OPEN` opens a long option; a `SELL` + `CLOSE` sells one you already hold; a `SELL` + `OPEN` writes a new short option.
***
## Example — buy 1 AAPL call to open (limit)
Buy to open 1 AAPL **$150 Call** expiring 2025-01-17, paying up to **$2.50** per contract.
```bash cURL theme={null}
curl -X POST https://api.aries.com/v1/orders \
-H "Authorization: Bearer YOUR_ACCESS_TOKEN" \
-H "Content-Type: application/json" \
-d '{
"tradingAccountId": "ACC123456",
"symbol": "AAPL",
"side": "BUY",
"type": "LIMIT",
"qty": "1",
"price": "2.50",
"timeInForce": "DAY",
"legs": [
{
"side": "BUY",
"ratioQty": "1",
"securityType": "OPT",
"putCall": "CALL",
"strikePrice": "150.00",
"maturity": "2025-01-17",
"positionEffect": "OPEN"
}
]
}'
```
### Example — sell to close 2 puts (limit)
```json Request body theme={null}
{
"tradingAccountId": "ACC123456",
"symbol": "AAPL",
"side": "SELL",
"type": "LIMIT",
"qty": "2",
"price": "3.10",
"timeInForce": "GTC",
"legs": [
{
"side": "SELL",
"ratioQty": "1",
"securityType": "OPT",
"putCall": "PUT",
"strikePrice": "145.00",
"maturity": "2025-02-21",
"positionEffect": "CLOSE"
}
]
}
```
### Example — market option order
```json theme={null}
{
"tradingAccountId": "ACC123456",
"symbol": "TSLA",
"side": "BUY",
"type": "MARKET",
"qty": "1",
"timeInForce": "DAY",
"legs": [
{
"side": "BUY",
"ratioQty": "1",
"securityType": "OPT",
"putCall": "CALL",
"strikePrice": "250.00",
"maturity": "2025-03-21",
"positionEffect": "OPEN"
}
]
}
```
Option orders accept only **`DAY`** or **`GTC`** time-in-force. `IOC` and `FOK` are rejected when a `legs` array is present.
***
## Response
```json theme={null}
{
"success": true,
"clOrdId": "CLIENT-100",
"status": "NEW",
"symbol": "AAPL",
"side": "BUY",
"qty": "1",
"cumQty": "0",
"leavesQty": "1",
"avgPrice": "0",
"transactTime": "2024-01-15T10:30:00Z"
}
```
***
## Watch it fill
Option order updates arrive on the same `account` topic as `account.order` events, but with `securityType: "OPTION"` and the `legs` echoed back:
```json account.order — single option filled theme={null}
{
"type": "event",
"timestamp": "2024-01-15T10:30:05Z",
"payload": {
"topic": "account",
"name": "account.order",
"target": "account:ACC123456",
"data": {
"ordId": "ORD-30001",
"clOrdId": "CLIENT-100",
"symbol": "AAPL",
"side": "BUY",
"type": "LIMIT",
"timeInForce": "DAY",
"qty": "1",
"price": "2.50",
"ordStatus": "FILLED",
"cumQty": "1",
"leavesQty": "0",
"avgPrice": "2.48",
"securityType": "OPTION",
"category": "OPTION",
"legs": [
{
"symbol": "AAPL",
"side": "BUY",
"ratioQty": "1",
"securityType": "OPTION",
"maturity": "2025-01-17",
"strikePrice": "150",
"positionEffect": "OPEN",
"putCall": "CALL"
}
],
"updatedAt": "2024-01-15T10:30:05Z"
}
}
}
```
For the connect/subscribe handshake see [Limit order → Listen for updates](/api-reference/orders/limit-order#listen-for-updates); for every field and status, the [Account Updates WebSocket](/websockets/account-updates) reference.
***
## Next steps
Combine 2–4 legs into spreads, calendars, and ratio strategies.
Look up the strikes and expirations available for an underlying.
The pricing and WebSocket mechanics carry over to options.
Interactive reference and full schema.