diff --git a/NEWS b/NEWS
index e21d9ece6..afbbe6a83 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-18:
+
+- Setting the 'posix' option now turns off the 'braceexpand' option, as brace
+  expansion is not specified by POSIX and potentially incompatible with sh
+  scripts. In addition, 'set -o posix' now turns on the 'letoctal' option
+  instead of controlling that behaviour directly. 'set +o posix' does the
+  reverse of these.
+
 2020-09-17:
 
 - In the vi and emacs line editors, repeat count parameters can now also be
diff --git a/src/cmd/ksh93/include/version.h b/src/cmd/ksh93/include/version.h
index 337d4a483..656a5ac9f 100644
--- a/src/cmd/ksh93/include/version.h
+++ b/src/cmd/ksh93/include/version.h
@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-17"
+#define SH_RELEASE	"93u+m 2020-09-18"
diff --git a/src/cmd/ksh93/sh.1 b/src/cmd/ksh93/sh.1
index b2166b943..ef222296c 100644
--- a/src/cmd/ksh93/sh.1
+++ b/src/cmd/ksh93/sh.1
@@ -6370,12 +6370,10 @@ to be evaluated.
 .B let
 only recognizes octal constants starting with
 .B 0
-if one of the
+when the
 .B set
-options
+option
 .B letoctal
-or
-.B posix
 is on.
 See
 .I "Arithmetic Evaluation"
@@ -6906,7 +6904,8 @@ is used, the variable
 is not unset first.
 .TP 8
 .B \-B
-Enable brace group expansion.  On by default.
+Enable brace group expansion. On by default, except
+if ksh is invoked as \fBsh\fR, \fBrsh\fR, or \fBpfsh\fR.
 .TP 8
 .B \-C
 Prevents redirection
@@ -7034,8 +7033,7 @@ The
 .B let
 command allows octal constants starting with
 .BR 0 .
-If the \fBposix\fR shell option is active,
-octals are recognized regardless of this option.
+On by default if ksh is invoked as \fBsh\fR, \fBrsh\fR, or \fBpfsh\fR.
 .TP 8
 .B markdirs
 All directory names resulting from file name generation have a trailing
@@ -7081,16 +7079,37 @@ will be the value of the last non-zero command
 to fail or zero if no command has failed.
 .TP 8
 .B posix
-Enable full POSIX standard compliance mode. This option
-is on by default if ksh is invoked as \fBsh\fR, \fBrsh\fR, or \fBpfsh\fR. It
-disables passing exported variables' attributes (such as integer or readonly) to a new ksh process through the environment,
-causes file descriptors > 2 to be left open when invoking another program,
-makes the \fB<>\fR redirection operator default to standard input,
-causes the \fBcommand\fR utility to disable declaration command properties of any declaration commands it invokes,
-skips the initialization of preset aliases upon invoking an interactive shell,
-disables a hack that makes \fBtest -t\fR (\fB[ -t ]\fR) equivalent to \fBtest -t 1\fR (\fB[ -t 1 ]\fR),
-enables octal numbers in \fBlet\fR shell arithmetic (see \fBletoctal\fR), and
-disables the \fB&>\fR redirection shorthand.
+Enables the POSIX standard mode for maximum compatibility with other compliant
+shells. At the moment that the \fBposix\fR option is turned on, it also turns
+on \fBletoctal\fR and turns off \fB\-B\fR/\fBbraceexpand\fR; the reverse is
+done when \fBposix\fR is turned back off. (These options can still be
+controlled independently in between.) Furthermore, the \fBposix\fR option
+is automaticaly turned on upon invocation if ksh is invoked as \fBsh\fR,
+\fBrsh\fR, or \fBpfsh\fR. In that case, or if the option is turned on by
+specifying \fB-o posix\fR on the invocation command line, the invoked shell
+will not set the preset aliases even if interactive, and will not import type
+attributes for variables (such as integer or readonly) from the environment.
+.RS 8
+.PP
+In addition, while on, the \fBposix\fR option
+.IP \[bu] 3
+disables exporting variable type attributes to the environment for other ksh
+processes to import;
+.IP \[bu]
+causes file descriptors > 2 to be left open when invoking another program;
+.IP \[bu]
+disables the \fB&>\fR redirection shorthand;
+.IP \[bu]
+makes the \fB<>\fR redirection operator default to redirecting standard input
+if no file descriptor number precedes it;
+.IP \[bu]
+causes the \fBcommand\fR utility to disable declaration command properties of
+any declaration commands it invokes;
+and
+.IP \[bu]
+disables a hack that makes \fBtest -t\fR (\fB[ -t ]\fR) equivalent to
+\fBtest -t 1\fR (\fB[ -t 1 ]\fR).
+.RE
 .TP 8
 .B privileged
 Same as
diff --git a/src/cmd/ksh93/sh/args.c b/src/cmd/ksh93/sh/args.c
index cb619abac..96220d3e7 100644
--- a/src/cmd/ksh93/sh/args.c
+++ b/src/cmd/ksh93/sh/args.c
@@ -289,9 +289,7 @@ int sh_argopts(int argc,register char *argv[], void *context)
 		}
 		argc--;
 	}
-	/* handling SH_INTERACTIVE and SH_PRIVILEGED has been moved to
-	 * sh_applyopts(), so that the code can be reused from b_shopt(), too
-	 */
+	/* SH_INTERACTIVE and SH_PRIVILEGED are handled in sh_applyopts() */
 	sh_applyopts(ap->sh,newflags);
 #if SHOPT_KIA
 	if(ap->kiafile)
@@ -343,6 +341,21 @@ void sh_applyopts(Shell_t* shp,Shopt_t newflags)
 			(shp->gd->userid==shp->gd->euserid && shp->gd->groupid==shp->gd->egroupid))
 				off_option(&newflags,SH_PRIVILEGED);
 	}
+	/* -o posix also switches -o braceexpand and -o letoctal */
+	if(!sh_isoption(SH_POSIX) && is_option(&newflags,SH_POSIX))
+	{
+#if SHOPT_BRACEPAT
+		off_option(&newflags,SH_BRACEEXPAND);
+#endif
+		on_option(&newflags,SH_LETOCTAL);
+	}
+	else if(sh_isoption(SH_POSIX) && !is_option(&newflags,SH_POSIX))
+	{
+#if SHOPT_BRACEPAT
+		on_option(&newflags,SH_BRACEEXPAND);
+#endif
+		off_option(&newflags,SH_LETOCTAL);
+	}
 	shp->options = newflags;
 }
 
diff --git a/src/cmd/ksh93/sh/arith.c b/src/cmd/ksh93/sh/arith.c
index 9ca43f0d3..cea06248a 100644
--- a/src/cmd/ksh93/sh/arith.c
+++ b/src/cmd/ksh93/sh/arith.c
@@ -390,10 +390,10 @@ static Sfdouble_t arith(const char **ptr, struct lval *lvalue, int type, Sfdoubl
 			char	lastbase=0, *val = xp, oerrno = errno;
 			lvalue->eflag = 0;
 			errno = 0;
-			if(shp->bltindata.bnode==SYSLET && !sh_isoption(SH_LETOCTAL) && !sh_isoption(SH_POSIX))
+			if(shp->bltindata.bnode==SYSLET && !sh_isoption(SH_LETOCTAL))
 			{	/*
 				 * Since we're running the "let" builtin, disable octal number processing by
-				 * skipping all initial zeros, unless the 'letoctal' or 'posix' option is on.
+				 * skipping all initial zeros, unless the 'letoctal' option is on.
 				 */
 				while(*val=='0' && isdigit(val[1]))
 					val++;
diff --git a/src/cmd/ksh93/sh/main.c b/src/cmd/ksh93/sh/main.c
index e1849a701..322a2b5ce 100644
--- a/src/cmd/ksh93/sh/main.c
+++ b/src/cmd/ksh93/sh/main.c
@@ -157,8 +157,11 @@ int sh_main(int ac, char *av[], Shinit_f userinit)
 		nv_putval(PS4NOD,e_traceprompt,NV_RDONLY);
 	path_pwd(shp,1);
 	iop = (Sfio_t*)0;
+	if(sh_isoption(SH_POSIX))
+		sh_onoption(SH_LETOCTAL);
 #if SHOPT_BRACEPAT
-	sh_onoption(SH_BRACEEXPAND);
+	else
+		sh_onoption(SH_BRACEEXPAND);
 #endif
 	if((beenhere++)==0)
 	{
diff --git a/src/cmd/ksh93/tests/options.sh b/src/cmd/ksh93/tests/options.sh
index 1e5dcf3bc..f0399a47c 100755
--- a/src/cmd/ksh93/tests/options.sh
+++ b/src/cmd/ksh93/tests/options.sh
@@ -533,9 +533,10 @@ print $'alias print=:\nprint foobar' > dotfile
 
 # ======
 # test that '-o posix' option (not having a letter) does not affect "$-" expansion
+# other than B = braceexpand
 (
 	command set +o posix 2>/dev/null
-	opt1=$-
+	opt1=${-/B/}
 	command set -o posix 2>/dev/null
 	opt2=$-
 	[[ $opt1 == "$opt2" ]]