diff --git a/hbrtl/register.go b/hbrtl/register.go
index daace7f..3ae1dfa 100644
--- a/hbrtl/register.go
+++ b/hbrtl/register.go
@@ -322,16 +322,30 @@ func RegisterRTL(vm *hbrt.VM) {
 
 		// Type checking (HB_IS*)
 		hbrt.Sym("HB_ISARRAY", hbrt.FsPublic, HbIsArray),
+		// Classic Clipper/Harbour aliases — `common.ch` #translate rules
+		// map these to HB_IS*. Registering the short names as direct
+		// symbols bypasses the PP altogether and makes them work even
+		// when code doesn't include common.ch.
+		hbrt.Sym("ISARRAY", hbrt.FsPublic, HbIsArray),
 		hbrt.Sym("HB_ISBLOCK", hbrt.FsPublic, HbIsBlock),
+		hbrt.Sym("ISBLOCK", hbrt.FsPublic, HbIsBlock),
 		hbrt.Sym("HB_ISCHAR", hbrt.FsPublic, HbIsChar),
 		hbrt.Sym("HB_ISSTRING", hbrt.FsPublic, HbIsString),
+		hbrt.Sym("ISCHARACTER", hbrt.FsPublic, HbIsString),
+		hbrt.Sym("ISMEMO", hbrt.FsPublic, HbIsString), // memo ≈ string for Five
 		hbrt.Sym("HB_ISDATE", hbrt.FsPublic, HbIsDate),
+		hbrt.Sym("ISDATE", hbrt.FsPublic, HbIsDate),
 		hbrt.Sym("HB_ISDATETIME", hbrt.FsPublic, HbIsDateTime),
 		hbrt.Sym("HB_ISLOGICAL", hbrt.FsPublic, HbIsLogical),
+		hbrt.Sym("ISLOGICAL", hbrt.FsPublic, HbIsLogical),
 		hbrt.Sym("HB_ISNUMERIC", hbrt.FsPublic, HbIsNumeric),
+		hbrt.Sym("ISNUMBER", hbrt.FsPublic, HbIsNumeric),
+		hbrt.Sym("ISNUMERIC", hbrt.FsPublic, HbIsNumeric),
 		hbrt.Sym("HB_ISOBJECT", hbrt.FsPublic, HbIsObject),
+		hbrt.Sym("ISOBJECT", hbrt.FsPublic, HbIsObject),
 		hbrt.Sym("HB_ISHASH", hbrt.FsPublic, HbIsHash),
 		hbrt.Sym("HB_ISNIL", hbrt.FsPublic, HbIsNil),
+		hbrt.Sym("ISNIL", hbrt.FsPublic, HbIsNil),
 		hbrt.Sym("HB_ISPOINTER", hbrt.FsPublic, HbIsPointer),
 
 		// OS/Environment
diff --git a/include/common.ch b/include/common.ch
new file mode 100644
index 0000000..22a2d61
--- /dev/null
+++ b/include/common.ch
@@ -0,0 +1,26 @@
+/*
+ * common.ch — Five compatibility shim for Harbour's common.ch
+ *
+ * Harbour ships #translate-based aliases for ISNIL / ISARRAY /
+ * ISNUMBER / ISCHARACTER / ISLOGICAL / ISDATE / ISBLOCK / ISMEMO /
+ * ISOBJECT. Five registers those as direct RTL symbols in
+ * hbrtl/register.go (each points at the same Go function as its
+ * HB_IS* twin), so they work without any preprocessor translation
+ * and without this include.
+ *
+ * This header stays as a stub so `#include "common.ch"` in ported
+ * Harbour code doesn't error. The DEFAULT / UPDATE #xcommand forms
+ * from Harbour's common.ch are not yet supported — use explicit
+ * `IF xVar == NIL ; xVar := default ; ENDIF` until the preprocessor's
+ * #xcommand marker handling is extended.
+ */
+
+#ifndef HB_COMMON_CH_
+#define HB_COMMON_CH_
+
+#define TRUE   .T.
+#define FALSE  .F.
+#define YES    .T.
+#define NO     .F.
+
+#endif