Accounting
Pacioli wrote down this system but he did not invent it, but he is still called the "Father of Accounting". The system he wrote had most of the accounting cycle as it is known today. He wrote about using journals and ledgers, and warned that a person should not go to sleep at night until the debits were the same as the credits. His ledger had accounts for assets (things that have value), liabilities (debts and loans that are to be paid to someone else), capital (money), income and expenses. He showed how to write year-end closing entries and suggested that a trial balance be used to prove that a ledger is balanced. His treatise (long essay) in the book also has info about other topics, such as accounting ethics and cost accounting.