Children with congenital deafblindness (CDB) have, similar as deaf and hearing children, an innate ability to acquire a language. It is known to be a challenge for children with CDB to acquire a first language. Most children with CDB do not learn to speak or sign and remain at a pre-linguistic level of language development. Observing non-linguistic and linguistic features is essential for pre-linguistic and early linguistic language development. This study used a systematic observation instrument to observe the non-linguistic and linguistic features of a child with CDB. The case study resulted in a qualitative analysis of communicative expressions and signs made by the child with CDB. BET's and gestures were expressed consistently and correctly in the context of the communication. The linguistic features of the signs expressed by the child did not match the features used in the conventional signs. Also, the child produced different phonological forms of one particular sign, making the signs of the child inconsistent. We discussed that inconsistent signs can lead to difficulty with the signs by the communication partner. Difficulty with readability of signs of the child by the communication partner may negatively influence the language development of the child. Different possible causes of the incorrect and inconsistent signs of the child were discussed, such as the inability of the child with CDB to access the linguistic features of the communication of the communication partner. Recommendations were given to support the language acquisition of children with CDB. Also, recommendations were given for further research.