A method by which a first client data store hosted by a first client device is synchronized with respect to a second client data store hosted by a second client device by synchronizing the two client data stores (10c 11c) with respect to a server data store hosted by a server device, the server device having an established connection with the two client devices, the two client data stores (10c 11c) each including various data fields, the method characterized by: synchronizing (32a 32b) the first client data store and the second client data store with the server data store respectively, so that the server data store includes all fields of the two client data stores (10c 11c); forming (23 39) structure information indicative of the structure of the two client data stores (10c 11c) by the server device based at least in part on the device type of the first client device and that of the second client device, the structure information being formed in respect of at least one data field of the first client data store for which the second client data store does not have either one corresponding data field or does not have two or more data fields that in combination correspond to the at least one data field; detecting, after the structure information has been formed, a use of the at least one data field in the first client data store by the server device analysing changes of the first client data store as reflected by the server data store; in response to the detection of a use of the at least one data field in the first client data store, setting a correspondence mapping the at least one data field in the first client data store in respect to the second client data store by the server device, so as to indicate that in subsequent synchronizations information included in the at least one data field is either not to be included in the second client data store or is to be included in one or more indicated data fields of the second client data store; and synchronizing the first client data store and the second client data store by the server device based at least in part on the set correspondence. synchronizing (32a 32b) the first client data store and the second client data store with the server data store respectively, so that the server data store includes all fields of the two client data stores (10c 11c); forming (23 39) structure information indicative of the structure of the two client data stores (10c 11c) by the server device based at least in part on the device type of the first client device and that of the second client device, the structure information being formed in respect of at least one data field of the first client data store for which the second client data store does not have either one corresponding data field or does not have two or more data fields that in combination correspond to the at least one data field; detecting, after the structure information has been formed, a use of the at least one data field in the first client data store by the server device analysing changes of the first client data store as reflected by the server data store; in response to the detection of a use of the at least one data field in the first client data store, setting a correspondence mapping the at least one data field in the first client data store in respect to the second client data store by the server device, so as to indicate that in subsequent synchronizations information included in the at least one data field is either not to be included in the second client data store or is to be included in one or more indicated data fields of the second client data store; and synchronizing the first client data store and the second client data store by the server device based at least in part on the set correspondence.