Information as to who was responsible for a particular painting is not always available and as a result such paintings are classed as by an anonymous artist. Usually experts will try to find any kind of evidence to prove the attribution of a painting, however insignificant this evidence might seem. However, often such data is not available. Artists only really began to sign their work during the Renaissance and accurate attribution is easier from the 16th or 17th century onwards. Nevertheless, there are still cases of anonymous works although from the 19th century onwards it becomes much more unusual.