The rules governing accession of movables to land are based on the dual principles of preservation of value and protection of third parties. Thus extinction of title prevents the attachment from being undone; and appearance determines legal effect. Neither principle has room for subjective intention or any other criterion unknown to outsiders. The rules of accession, therefore, should be based on objective criteria. A retention of title clause, for example, should not be allowed to postpone accession. None the less in the legal systems described below there is considerable variation both in the rules of accession and in the right of severance (ius tollendi). This suggests that any attempt at harmonising security rights in European private law is likely to fail unless there is also harmonisation of the rules of accession and the right of severance. This article will be published in two parts. The second part will appear in the next issue.