+@cindex node-id
+@cindex entity-number
+@cindex message
+@cindex root-entity
+
+Structure of a MIME message is tree.@refill
+
+In the tree, root node is the entity indicates all of the message. In
+this document, it is called @strong{root-entity} or @strong{message}.
+In FLIM, it is indicated by buffer local variable
+@code{mime-message-structure}.@refill
+
+Each entity except root-entity has a parent. An entity may have
+children. We can indicate an entity by relative position from a base
+entity, based on the parent-child relationship.@refill
+
+In addition, we can indicate an entity by absolute position of the
+message.@refill
+
+Each entity, which is a node of the tree, can be numbered by
+depth and left-to-right order of the depth.
+@example
+
+ +-------+
+ | nil |
+ +---+---+
+ +-------------------+-------------------+
+ +-+-+ +-+-+ +-+-+
+ | 0 | | 1 | | 2 |
+ +-+-+ +-+-+ +-+-+
+ | +---------+---------+ |
+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
+ | 0.0 | | 1.0 | | 1.1 | | 1.2 | | 2.0 |
+ +-----+ +-----+ +-----+ +-----+ +-----+
+@end example
+
+Namely, if depth of a node is n, the node has a node-number, which is
+consists of n integers. In this document, it is called
+@strong{entity-number}. An entity-number is represented by list of
+integer, like @code{(1 2 3)}.@refill
+
+mime-entity has also @strong{node-id}. A node-id is represented by
+reversed list of entity-number. For example, node-id corresponding with
+1.2.3 is @code{(3 2 1)}.@refill
+
+Each entity can be indicated by entity-number or node-id in
+@code{mime-message-structure}.