diff --git a/Makefile.inc1 b/Makefile.inc1
index 84d2f9db24aa..d9d8010d3039 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -11,7 +11,7 @@
 #	-DNO_PORTSUPDATE do not update ports in ${MAKE} update
 #	-DNO_DOCUPDATE do not update doc in ${MAKE} update
 #	LOCAL_DIRS="list of dirs" to add additional dirs to the SUBDIR list
-#	TARGET="arch" to crossbuild world to a different arch
+#	TARGET="machine" to crossbuild world for a different machine type
 
 #
 # The intended user-driven targets are:
@@ -95,17 +95,36 @@ OSRELDATE!=	awk '/^\#define[[:space:]]*__FreeBSD_version/ { print $$3 }' \
 OSRELDATE=	0
 .endif
 .endif
-.if defined(TARGET) && !defined(TARGET_ARCH)
-TARGET_ARCH=${TARGET:S/pc98/i386/}
+
+# Guess machine architecture from machine type, and vice versa.
+.if !defined(TARGET_ARCH) && defined(TARGET)
+TARGET_ARCH=	${TARGET:S/pc98/i386/}
+.elif !defined(TARGET) && defined(TARGET_ARCH) && \
+    ${TARGET_ARCH} != ${MACHINE_ARCH}
+TARGET=		${TARGET_ARCH}
 .endif
-TARGET_ARCH?=	${MACHINE_ARCH}
-.if ${TARGET_ARCH} == ${MACHINE_ARCH}
+# Otherwise, default to current machine type and architecture.
 TARGET?=	${MACHINE}
+TARGET_ARCH?=	${MACHINE_ARCH}
+
+KNOWN_ARCHES?=	amd64 arm i386 i386/pc98 ia64 powerpc sparc64
+.if ${TARGET} == ${TARGET_ARCH}
+_t=		${TARGET}
+.else
+_t=		${TARGET_ARCH}/${TARGET}
+.endif
+.for _t in ${_t}
+.if empty(KNOWN_ARCHES:M${_t})
+.error Unknown target ${TARGET_ARCH}:${TARGET}.
+.endif
+.endfor
+
+.if ${TARGET} == ${MACHINE}
 TARGET_CPUTYPE?=${CPUTYPE}
 .else
-TARGET?=	${TARGET_ARCH}
 TARGET_CPUTYPE?=
 .endif
+
 .if !empty(TARGET_CPUTYPE)
 _TARGET_CPUTYPE=${TARGET_CPUTYPE}
 .else