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+!!! abstract This area of CiviCRM code and documentation is a work-in-progress. Not all features will be documented and the core code underlying this area may change from version to version.
+
+This file exists to give an overview of the Financial entities involved in recording Financial data in CiviCRM.
+
+You should only interact with the Financial entities through the relevant apis - generally Order & Payment apis.
+
+It is a work in progress!
+
+The original file for tracking these is [on the old CiviCRM Wik](https://wiki.civicrm.org/confluence/display/CRM/CiviAccounts+Data+Flow).
+
+## Financial Accounts
+
+These are accounting financial accounts. They have fields intended to help map to an accounting package that
+are not used functionally in CiviCRM but are available in various reports.
+
+* accounting_code (free-form text)
+* account_type_code (free-form text)
+* financial_account_type_id (linked to the option group with the name financial_account_type)
+* contact id (not used in core but can be used in extensions - e.g. this is used in the [connectors](https://github.com/eileenmcnaughton/nz.co.fuzion.connectors) extension to link some accounts to one Xero account & others to a different one).
+* is_header_account
+
+In addition there are some functional fields that affect how tax is handled:
+
+* tax_rate
+* is_deductible
+* is_tax
+
+![Example financial accounts](/img/financial/FinancialAccount.png)
+
+## Financial Transactions
+
+Movements into or out of financial accounts are represented by financial transactions in the civicrm_financial_trxn table.
+
+Each financial transaction has a `from_account_id` and a `to_account_id`. These represent movements between financial accounts.
+
+They also have various informative fields:
+
+* trxn_date
+* fee_amount, net_amount, total_amount
+* currency
+* is_payment (is this a payment financial transaction - see section on [payments](#payments))
+* payment_instrument_id (links to payment_instrument option group - e.g check, credit card)
+* payment_processor_id (when a payment processor has processed the payment)
+* card_type_id, pan_truncation, check_number - these are additional bits of information gathered about the payment
+* trxn_id, trxn_result_code, order_reference - these are all generated by the payment processor (when there is one) and
+stored in CiviCRM when available. The trxn_id is the processor reference for the gateway. Some processors have references
+at the order level. Even though these are at the order level there could be more than one per order as more than one.
+payment with more than one processor might be paid against an order (contribution), hence they are stored on the financial transaction
+* status_id - the intent is that failed as well as successful payments should be stored (and appropriately filtered in bookkeeping reports)
+
+## Payments
+
+Some financial transactions are payments and these are marked with the `is_payment` field on that table. Payments in CiviCRM are a subset
+of the rows in `civicrm_financial_trxn`. You should use the Payment API to interact with these payment financial transactions.
+
+Importantly payments will end up moving money ***to*** an account where payments are collected - ie a Bank account
+or a Payment Processor account (which in the real world may be an actual account or just a clearing house for nightly payments.)
+Negative payments (or refunds) will have the bank or processor account on the ***from*** side of the transaction.
+
+## Financial Items
+
+Financial items exist to track how much has been paid against the various line items within an order / contribution.
+In an accounting system this might be called credit-matching.
diff --git a/docs/financial/OrderAPI.md b/docs/financial/orderAPI.md
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rename to docs/financial/orderAPI.md
diff --git a/docs/financial/PaymentAPI.md b/docs/financial/paymentAPI.md
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rename to docs/financial/paymentAPI.md
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diff --git a/mkdocs.yml b/mkdocs.yml
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@@ -32,8 +32,9 @@ pages:
   - Release Process: core/release-process.md
 - Financial:
   - Overview: financial/overview.md
-  - Order API: financial/OrderAPI.md
-  - Payments API: financial/PaymentAPI.md
+  - Order API: financial/orderAPI.md
+  - Payment API: financial/paymentAPI.md
+  - Entities: financial/financialentities.md
 - Extensions:
   - Basics: extensions/index.md
   - Packaging Options: extensions/packaging.md