# Knowledge base

Source: <https://docs.gjold.com/learn>

How card payments work, and the rules that card schemes, acquirers, and banks set for them. The API docs say what the gjold API does; this says why, and what happens outside it.

Most of what is here is not ours to decide. Visa and Mastercard set rules for every merchant, an acquirer adds its own, and the payer's bank decides each payment. The rules change, so each page with numbers on it says when it was last reviewed. If your acquirer's contract says otherwise, the contract applies.

## Payments

- [How payments work](https://docs.gjold.com/learn/payments): who takes part in a card payment, and what happens from the payer's click to the money in your account.
- [Authorizations](https://docs.gjold.com/learn/authorizations): what an authorization holds, how long it lasts, and what happens when it runs out.
- [Captures and refunds](https://docs.gjold.com/learn/captures-and-refunds): when you may take the money, and how to give it back.
- [Chargebacks](https://docs.gjold.com/learn/chargebacks): how a payer disputes a payment through their bank, and what you can do about it.

## Payment methods

- [Payment methods](https://docs.gjold.com/learn/payment-methods): cards and their brands, Apple Pay, and MobilePay, and which of them can be saved.
- [The payment window](https://docs.gjold.com/learn/payment-window): what the payer sees on a payment link, from the amount to the confirmation.

## Security

- [3-D Secure](https://docs.gjold.com/learn/3-d-secure): strong customer authentication, which the payer's bank asks for, and which payments are exempt.
- [PCI DSS](https://docs.gjold.com/learn/pci-dss): the card industry's security standard, and what it asks of a shop that uses payment links.

## Subscriptions

- [Stored credentials](https://docs.gjold.com/learn/stored-credentials): saving a payer's card, and charging it later without them.
- [Declined charges](https://docs.gjold.com/learn/declined-charges): why a charge is declined, and how often the card schemes let you try again.

## In the API docs

The [gjold API docs](https://docs.gjold.com/) describe each request, its fields, and its errors, and the [implementation notes](https://docs.gjold.com/notes) how to build an integration that holds up. They link here where a rule outside the API matters.