Blockchain systems have the potential to decentralise many traditionally centralised systems. However, scalability remains a key challenge. Without a horizontally scalable solution, where performance increases by adding more nodes to the system, blockchain systems remain unsuitable for ubiquitous use. We design a novel blockchain system called Checo. Each node in our system maintains a personal hash chain, which only stores transactions that the node is involved in. A consensus is reached on special blocks called checkpoint blocks rather than on all the transactions. Checkpoint blocks are effectively a hash pointer to the personal hash chains; thus a single checkpoint block may represent an arbitrarily large set of transactions. We introduce a validation protocol so that any node can check the validity of any transaction. Since it is a point-to-point protocol, we achieve horizontal scalability. We analytically evaluate our system and proof a number of highly desirable correctness properties. Further, we give a free and open-source implementation of Checo and evaluate it experimentally. Our results show a strong indication of horizontal scalability.