# Conversations The WhatsApp API pricing model is based on conversations. This means that businesses pay per conversation, which includes all messages interchanged within a 24-hour period. There are two types of conversations on the WhatsApp Business API, each with different rates: * **User-initiated**: A conversation that initiates in response to a user message. Any time a business replies to a user within the 24-hour customer service window, that message will be associated with a user-initiated conversation. * **Business-initiated**: A conversation that initiates from a business sending a user a message outside the 24-hour customer service window. The conversation starts when the first business message in a conversation is delivered. However, the conversation type will be defined once a message has been sent by the business. ### Sessions All conversations are measured in 24-hour increments or “sessions”, as we’ll refer to them here. A session starts whenever the first message is sent by a business, either in response to a user inquiry or if a business-initiated message template is delivered. ### Policy The policy requires the use of a template message whenever sending a message to a user more than 24 hours after their last message. So, a new session begins. If you send a free-form message outside the customer support window, you are not following the policy, so the delivery of this message will fail. However, it opens a business-initiated conversation that might be charged. If the user sends a message back (replies) within 24 hours of the failed delivery, this conversation will be charged as business-initiated. Because, although the free-form message was not delivered, it was sent by the business. ### Customer Service Window Please note that **conversations rules are different and independent from customer service windows**, which are used to understand if the business can send a free-form message or a template message. See the example below for more details: