Here, for example, is a part of the formal DTD for the register example given informally above.
<!ELEMENT register - - (entry+)>
<!ELEMENT entry - o (name, trade, relation?)>
<!ELEMENT name - - (#PCDATA)>
These three lines are examples of SGML declarations: each defines or declares a name for an element and what its content should be. The details of the syntax need not detain us; note only that each declaration (like everything else in SGML) is explicitly delimited, in this case by a symbol marking the start of a declaration (the “[`<!]”)
(Burnard 2014,§ 5.1)