# Existing tags and data layers

Kontala can read what your site already tells other tools, so you do not need to re-tag your pages.

## Data layers

Choose which data layers to capture on the **Data layer** page, under **Capture rules**.
Kontala reads these out of the box:

| Data layer | What is read |
| --- | --- |
| Google Tag Manager | `window.dataLayer`, which gtag.js and Piwik PRO use too |
| Tealium iQ | `window.utag_data`, and calls to `utag.view`, `utag.link` and `utag.track` |
| Adobe Client Data Layer | `window.adobeDataLayer` |
| W3C digitalData | `window.digitalData` |
| Commanders Act | `window.tc_vars` |
| Matomo Tag Manager | `window._mtm` |
| Shopify | `window.ShopifyAnalytics.meta` and the store's details |

You can also add up to 10 custom sources: any other global object your site keeps.

## Analytics tools

Kontala can also read the calls your site makes to these 11 analytics tools:

Adobe Analytics, Adobe Web SDK, Amplitude, Contentsquare, Hotjar, Matomo, Meta Pixel, Microsoft Clarity, Mixpanel, Segment and Yandex Metrica.

It reads their events, page calls and identify calls, including calls your site queued before the tool's library loaded.
Google Analytics `gtag()` commands arrive through `window.dataLayer`, so they are read with the Google Tag Manager data layer.

## How a page's data layer is captured

With every page view, the tag takes one snapshot of the data layer, by default a second after the page opens.
Every event your site pushes to the data layer is captured as well.

## Capture rules

For the first 30 days, everything you have not discarded is captured, so you can see what your site sends.
After that, only the fields you have mapped are captured.
Turn on each property's **Data layer** card to read its pages.

## Standard fields

Map your fields to Kontala's standard fields on the **Data layer** page, under **Standard fields**.
Standard fields have the same name on every site, such as `k_order_value` for an order's value, `k_order_transaction_id` for its id, and `k_cart_total` for a cart's total.
Fields that only your organisation uses are kept as custom fields, starting `dl_`.

Press **Publish** to send your mappings to Analytics, where they apply to all of your history.