> ## Documentation Index > Fetch the complete documentation index at: https://harnesscalendar.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Key concepts > Learn the core ideas behind how Harness fits together. A quick map of how Harness fits together. The rest of the docs build on these terms. ## Your studio and its people Your **studio** is your whole workspace: your calendar, your students, and your books. You run one. A **student** is someone you teach. A **billing account** is who pays for them. Often that's the same person; for families it isn't: the Reyes kids each have a student profile, but one account, so the family gets one invoice a month. **Contacts** are the people attached to a student: some receive lesson emails, and an account's contacts receive its invoices. See [Accounts and students](/docs/students/accounts-and-students). ## Your calendar Everything you do sits on the **calendar** as an **event**. An event is either a **lesson** (which you teach, mark **attendance** for, and can bill) or any other entry, like a gig, a rehearsal, or a personal block. A **recurring** event repeats on a schedule, so a weekly lesson is one event, not fifty. See [Navigate your calendar](/docs/calendar/navigate-your-calendar). ## Letting people book you **Scheduling links** are shareable pages where students or families book time from your open availability; a **reschedule link** lets someone move an existing lesson. See [Scheduling links](/docs/calendar/scheduling-links). **Annotations** mark patterns on your calendar, like your teaching hours, so you can see them at a glance. They're visual only: nothing is blocked or booked through them. See [Annotations](/docs/calendar/annotations). ## Getting paid Money flows one direction: * A **charge** (what's owed) comes from a lesson's attendance, from a **recurring charge** like monthly tuition, or from a paid event like a gig. * An **invoice** collects charges into a bill sent to the account. Each account bills on its own **cycle**, and Harness can draft or even send the invoice itself. * A **payment** settles it. A **credit** (goodwill on an account) lowers a future bill, and a **refund** returns money. * An account's **balance** is what it owes, or holds in credit. In practice: mark Tuesday's lesson attended and a \$40 charge lands on the account; at the account's cycle close one invoice collects its charges; a payment clears it. See [How billing works](/docs/billing/how-billing-works) and [Automated billing](/docs/billing/automated-billing). You never compute a balance yourself. Harness derives every total from the charges and payments on record. ## Your books Beyond billing, Harness tracks your **expenses** and turns your income and costs into a **Profit & Loss** statement for tax time. See [Reports](/docs/accounting/reports). ## Your plan Harness has two plans: **Hobby** (free, up to 5 students) and **Professional** (no student limit, plus online payments, booking links, and automated billing). See [Plans and limits](/docs/settings/plans-and-limits) for what sits where, and [Manage your plan](/docs/settings/manage-your-plan) to change yours.