The "Bookkeeping Ontology", a conceptual model being based on the REA model and CIDOC CRM, is developed in an ontology engineering process in which historians, software developers and digital humanists were involved. It formalizes the interpretation of a transaction (bk:Transaction) as combination of transfers (bk:Transfer) of measurable objects (bk:Measurable) from one accounting object (bk:EconomicAgent) to another. bk:EconomicAgent defines an abstract class which unites bookkeeping categories (bk:Account, e.g. a cash account) and actors (individual bk:Party, e.g. Washington or an unknown group of individuals bk:Group, e.g. four farmers). Its physical representation in a historical source is an entry in a written accounting record (bk:entry). The bk:Entry is an information fragment of a bk:Transaction often naming only one party, while the other party is implicit in the textual context of the entry. Further information on the temporal (bk:when), spatial (bk:where) dimension of a bk:Transaction as well as the status (bk:status) of it (e.g. "partly paid", "settled"), can be expressed optionally. In regard to a research question a bk:Transaction can be assigned to a specific context. Every transaction consists (bk:consistsOf) of at least one transfer (bk:Transfer). A single bk:Transfer describes the action of transferring a bk:Measurable in one direction (bk:from or bk:to). bk:Measurable is defined as everything that can be quantified. It has subclasses for economic goods (bk:EconomicGood, as labor: bk:Service or as physical things: bk:Commodity) and money (bk:Money). bk:Measurable is describe by its quantity (bk:quantity) and the unit of calculation (bk:unit). The bk:Entry is described by the transcription fragment of the original source (bk:text). bk:EconomicGoods can be categorized (what is measured) and can be assigned a price. A bk:Transfer can be carried out by (bk:by) someone who conducts the transfer process in place of the business partner (bk:Agent). When writing it down into the ledger, accounting categories (bk:debit and bk:credit) are coded optionally.