# Line number

Source: https://developer.avalara.com/avatax-dm-combined-erp/sales-tax-badge/transactions/cert-line-number/

# Line number

**Element type: Required**

**Endpoints/models used:** [LineItemModel](https://developer.avalara.com/api-reference/avatax/rest/v2/models/LineItemModel/)

Your integration must provide a way for users to enter a unique line number for each line in a transaction. This is an optional field for users. AvaTax will automatically number lines on an invoice starting with 1.

A line number is added to your application using the`number` field in the transaction's [LineItemModel](https://developer.avalara.com/api-reference/avatax/rest/v2/models/LineItemModel/).

```
{    "lines": [        {            "number": "1",            "quantity": 1,            "amount": 100,            "taxCode": "PS081282",            "itemCode": "Y0001",            "description": "Yarn"        }    ],  ...}
```

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