Index: doc/filename.n
==================================================================
--- doc/filename.n
+++ doc/filename.n
@@ -45,11 +45,12 @@
 components are separated by slashes.  Path names may be relative or
 absolute, and file names may contain any character other than slash.
 The file names \fB\&.\fR and \fB\&..\fR are special and refer to the
 current directory and the parent of the current directory respectively.
 Multiple adjacent slash characters are interpreted as a single
-separator.  Any number of trailing slash characters at the end of a
+separator, except for the first double slash \fB//\fR in absolute paths.
+Any number of trailing slash characters at the end of a
 path are simply ignored, so the paths \fBfoo\fR, \fBfoo/\fR and
 \fBfoo//\fR are all identical, and in particular \fBfoo/\fR does not
 necessarily mean a directory is being referred.
 .RS
 .PP

Index: generic/tclFileName.c
==================================================================
--- generic/tclFileName.c
+++ generic/tclFileName.c
@@ -411,35 +411,22 @@
 	     * Paths that begin with / are absolute.
 	     */
 
 	    if (path[0] == '/') {
 		++path;
-#if defined(__CYGWIN__) || defined(__QNX__)
 		/*
 		 * Check for "//" network path prefix
 		 */
 		if ((*path == '/') && path[1] && (path[1] != '/')) {
 		    path += 2;
 		    while (*path && *path != '/') {
 			++path;
 		    }
-#if defined(__CYGWIN__)
-		    /* UNC paths need to be followed by a share name */
-		    if (*path++ && (*path && *path != '/')) {
-			++path;
-			while (*path && *path != '/') {
-			    ++path;
-			}
-		    } else {
-			path = origPath + 1;
-		    }
-#endif
-		}
-#endif
+		}
 		if (driveNameLengthPtr != NULL) {
 		    /*
-		     * We need this addition in case the QNX or Cygwin code was used.
+		     * We need this addition in case the "//" code was used.
 		     */
 
 		    *driveNameLengthPtr = (path - origPath);
 		}
 	    } else {
@@ -654,32 +641,19 @@
 
     TclNewObj(result);
     if (*path == '/') {
 	Tcl_Obj *rootElt;
 	++path;
-#if defined(__CYGWIN__) || defined(__QNX__)
 	/*
 	 * Check for "//" network path prefix
 	 */
 	if ((*path == '/') && path[1] && (path[1] != '/')) {
 	    path += 2;
 	    while (*path && *path != '/') {
 		++path;
 	    }
-#if defined(__CYGWIN__)
-	    /* UNC paths need to be followed by a share name */
-	    if (*path++ && (*path && *path != '/')) {
-		++path;
-		while (*path && *path != '/') {
-		    ++path;
-		}
-	    } else {
-		path = origPath + 1;
-	    }
-#endif
-	}
-#endif
+	}
 	rootElt = Tcl_NewStringObj(origPath, path - origPath);
 	Tcl_ListObjAppendElement(NULL, result, rootElt);
 	while (*path == '/') {
 	    ++path;
 	}

Index: tests/cmdAH.test
==================================================================
--- tests/cmdAH.test
+++ tests/cmdAH.test
@@ -437,23 +437,23 @@
     file dirname /foo
 } /
 test cmdAH-8.14 {Tcl_FileObjCmd: dirname} testsetplatform {
     testsetplatform unix
     file dirname //foo
-} /
+} //foo
 test cmdAH-8.15 {Tcl_FileObjCmd: dirname} testsetplatform {
     testsetplatform unix
     file dirname //foo/bar
-} /foo
+} //foo
 test cmdAH-8.16 {Tcl_FileObjCmd: dirname} testsetplatform {
     testsetplatform unix
     file dirname {//foo\/bar/baz}
-} {/foo\/bar}
+} {//foo\/bar}
 test cmdAH-8.17 {Tcl_FileObjCmd: dirname} testsetplatform {
     testsetplatform unix
     file dirname {//foo\/bar/baz/blat}
-} {/foo\/bar/baz}
+} {//foo\/bar/baz}
 test cmdAH-8.18 {Tcl_FileObjCmd: dirname} testsetplatform {
     testsetplatform unix
     file dirname /foo//
 } /
 test cmdAH-8.19 {Tcl_FileObjCmd: dirname} testsetplatform {
@@ -581,11 +581,11 @@
     file tail /foo
 } foo
 test cmdAH-9.14 {Tcl_FileObjCmd: tail} testsetplatform {
     testsetplatform unix
     file tail //foo
-} foo
+} {}
 test cmdAH-9.15 {Tcl_FileObjCmd: tail} testsetplatform {
     testsetplatform unix
     file tail //foo/bar
 } bar
 test cmdAH-9.16 {Tcl_FileObjCmd: tail} testsetplatform {

Index: tests/fileName.test
==================================================================
--- tests/fileName.test
+++ tests/fileName.test
@@ -199,11 +199,11 @@
     file split ../..
 } {.. ..}
 test filename-4.13 {Tcl_SplitPath: unix} {testsetplatform} {
     testsetplatform unix
     file split //foo
-} "/ foo"
+} "//foo"
 test filename-4.14 {Tcl_SplitPath: unix} {testsetplatform} {
     testsetplatform unix
     file split foo//bar
 } {foo bar}
 test filename-4.15 {Tcl_SplitPath: unix} {testsetplatform} {
@@ -436,18 +436,18 @@
     file join a . ./~b
 } {a/./~b}
 test filename-7.17 {Tcl_JoinPath: unix} {testsetplatform} {
     testsetplatform unix
     file join //a b
-} "/a/b"
+} "//a/b"
 test filename-7.18 {Tcl_JoinPath: unix} {testsetplatform} {
     testsetplatform unix
     file join /// a b
 } "/a/b"
 test filename-7.19 {[Bug f34cf83dd0]} {
     file join foo //bar
-} /bar
+} //bar
 
 test filename-9.1 {Tcl_JoinPath: win} {testsetplatform} {
     testsetplatform win
     file join a b
 } {a/b}

Index: tests/fileSystem.test
==================================================================
--- tests/fileSystem.test
+++ tests/fileSystem.test
@@ -377,17 +377,17 @@
     testPathEqual [file norm /../..] [file norm /]
 } ok
 test filesystem-1.51.1 {file normalisation .. beyond root (Bug 1379287)} {
     testPathEqual [file norm /../../] [file norm /]
 } ok
-test filesystem-1.52 {bug f9f390d0fa: file join where strep is not canonical} -constraints unix -body {
-    set x //foo
+test filesystem-1.52 {bug f9f390d0fa: file join where strep is not canonical} -body {
+    set x ///foo
     file normalize $x
     file join $x bar
 } -result /foo/bar
 test filesystem-1.52.1 {bug f9f390d0fa: file join where strep is not canonical} -body {
-    set x //foo
+    set x ///foo
     file normalize $x
     file join $x
 } -result /foo
 test filesystem-1.53 {[Bug 3559678] - normalize when tail is empty} {
   string match */ [file normalize [lindex [glob -dir [pwd] {{}}] 0]]