Index: lm.Rd
===================================================================
--- lm.Rd	(revision 81416)
+++ lm.Rd	(working copy)
@@ -33,7 +33,9 @@
     typically the environment from which \code{lm} is called.}
 
   \item{subset}{an optional vector specifying a subset of observations
-    to be used in the fitting process.}
+    to be used in the fitting process. (See additional details about how
+    this argument interacts with data-dependent bases in the
+    \sQuote{Details} section of the \code{\link{model.frame}} documentation.)
 
   \item{weights}{an optional vector of weights to be used in the fitting
     process.  Should be \code{NULL} or a numeric vector.
Index: model.frame.Rd
===================================================================
--- model.frame.Rd	(revision 81416)
+++ model.frame.Rd	(working copy)
@@ -38,7 +38,9 @@
   \item{subset}{a specification of the rows to be used: defaults to all
     rows. This can be any valid indexing vector (see
     \code{\link{[.data.frame}}) for the rows of \code{data} or if that is not
-    supplied, a data frame made up of the variables used in \code{formula}.}
+    supplied, a data frame made up of the variables used in
+    \code{formula}. (See additional details about how this argument
+    interacts with data-dependent bases under \sQuote{Details} below.)
 
   \item{na.action}{how \code{NA}s are treated.  The default is first,
     any \code{na.action} attribute of \code{data}, second
@@ -103,6 +105,12 @@
   character variable is found, it is converted to a factor (as from \R
   2.10.0).
 
+  Because variables in the formula are evaluated before rows are dropped
+  based on \code{subset}, the characteristics of data-dependent bases
+  such as orthogonal polynomials (i.e. from terms using
+  \code{\link{poly}}) or splines will be computed based on the full data
+  set rather than the subsetted data set.
+
   Unless \code{na.action = NULL}, time-series attributes will be removed
   from the variables found (since they will be wrong if \code{NA}s are
   removed).