[34] Contract balances

Contract assets stood at €519.1 million (December 31, 2020: €172.1 million); most of this amount, €504.2 million (December 31, 2020: €162.2 million), was attributable to goods and services provided in the project business that have not yet been billed.

Of the contract liabilities, €675.0 million was attributable to project business contracts with a net debit balance due to customers (December 31, 2020: €439.2 million) and €179.8 million to prepayments received from customers (December 31, 2020: €111.6 million). The revenue recognized in the reporting period that was included in the contract liability balance at the beginning of the period amounted to €518.8 million (2020: €471.8 million).