[34] Contract balances

Contract assets stood at €172.1 million (December 31, 2019: €150.2 million); most of this amount, €162.2 million (December 31, 2019: €143.6 million), was attributable to project business contracts.

Of the contract liabilities, €439.2 million was attributable to project business contracts with a net debit balance due to customers (December 31, 2019: €416.8 million) and €111.6 million to prepayments received from customers (December 31, 2019: €88.1 million). They relate to services that are still to be provided but for which prepayments from customers have been received. Contract liabilities are recognized as revenue as soon as the contractual goods and services have been provided. The revenue recognized in the reporting period that was included in the contract liability balance at the beginning of the period amounted to €471.8 million (2019: €468.7 million). Prepayments received from customers came to €645.5 million (2019: €549.6 million).