# Line number

Source: https://developer.avalara.com/vat-ecom/transactions/certification-requirements/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 field is optional for users. If not specified, 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/).

[![Closed](https://avalara-devdocs-prod.mcoutput.com/vat-ecom/Skins/Default/Stylesheets/Images/transparent.gif)View example](javascript:void\(0\);)

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