+.SS User-Defined Capabilities
+.
+The preceding section listed the \fIpredefined\fP capabilities.
+They deal with some special features for terminals no longer
+(or possibly never) produced.
+Occasionally there are special features of newer terminals which
+are awkward or impossible to represent by reusing the predefined
+capabilities.
+.PP
+\fBncurses\fP addresses this limitation by allowing user-defined capabilities.
+The \fB@TIC@\fP and \fB@INFOCMP@\fP programs provide
+the \fB\-x\fP option for this purpose.
+When \fB\-x\fP is set,
+\fB@TIC@\fP treats unknown capabilities as user-defined.
+That is, if \fB@TIC@\fP encounters a capability name
+which it does not recognize,
+it infers its type (boolean, number or string) from the syntax
+and makes an extended table entry for that capability.
+The \fBuse_extended_names\fP function makes this information
+conditionally available to applications.
+The ncurses library provides the data leaving most of the behavior
+to applications:
+.bP
+User-defined capability strings whose name begins
+with \*(``k\*('' are treated as function keys.
+.bP
+The types (boolean, number, string) determined by \fB@TIC@\fP
+can be inferred by successful calls on \fBtigetflag\fP, etc.
+.bP
+If the capability name happens to be two characters,
+the capability is also available through the termcap interface.
+.PP
+While termcap is said to be extensible because it does not use a predefined set
+of capabilities,
+in practice it has been limited to the capabilities defined by
+terminfo implementations.
+As a rule,
+user-defined capabilities intended for use by termcap applications should
+be limited to booleans and numbers to avoid running past the 1023 byte
+limit assumed by termcap implementations and their applications.
+In particular, providing extended sets of function keys (past the 60
+numbered keys and the handful of special named keys) is best done using
+the longer names available using terminfo.