Abstract: Data structure and methods associating an arbitrary list of properties with user interface items on a graphical user interface display. Each property has a tag with which an application program can reference it, and arbitrary data which can be written, read and modified by the application program. Property lists for each user interface item are stored in in-memory pools of property lists. Each pool includes a header region having a variable number of index slots, each slot which is currently being used pointing to a respective property list in a property list region of the pool. Unused slots are maintained on a linked list of index slots. When a property list is to be added or removed from a pool, the maintenance routines grow or shrink the pool only by fixed increments. Each property list includes a variable number of property entries each having a tag field and a data field.