diff --git a/docs/mm.1 b/docs/mm.1
index 9aa69f5..a2419da 100644
--- a/docs/mm.1
+++ b/docs/mm.1
@@ -4,7 +4,7 @@
 .\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
 .\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
 .\"
-.TH MM 1 2024-07-14 "Harakit X.X.X"
+.TH MM 1 2025-11-01 "Harakit X.X.X"
 .SH NAME
 mm \(en middleman
 .\"
@@ -22,7 +22,9 @@ Catenate input files and write them to the start of each output file or stream.
 .SH OPTIONS
 
 .IP \fB-a\fP
-Opens outputs for appending rather than updating.
+Opens outputs for appending rather than updating. Implies the
+.B -t
+option.
 .IP \fB-e\fP
 Use the standard error as an output.
 .IP \fB-t\fP
@@ -59,6 +61,10 @@ unncessary.
 .\"
 .SH HISTORY
 
+The \fB-t\fP option was originally the reverse functionality, as the default
+behavior was to overwrite outputs. This was deemed counterintuitive and the
+default behavior was changed.
+
 This utility originally had a \fB-u\fP option for compatibility with cat(1p),
 but this option is almost always default behavior in practice; therefore, it
 was dropped both for simplicity and to expand the
diff --git a/src/mm.rs b/src/mm.rs
index 9ad4cb8..42a4fad 100644
--- a/src/mm.rs
+++ b/src/mm.rs
@@ -168,7 +168,7 @@ fn main() -> ExitCode {
 
 		let options = File::options()
 			/* don’t truncate if -t is specified, append if -a is specified */
-			.truncate(t)
+			.truncate(!a && t)
 			.append(a)
 			/* enable the ability to create and write to files */
 			.create(true)
diff --git a/tests/mm.mk b/tests/mm.mk
index 11f83fa..1413cbd 100755
--- a/tests/mm.mk
+++ b/tests/mm.mk
@@ -1,10 +1,12 @@
-# Copyright (c) 2024 Emma Tebibyte <emma@tebibyte.media>
+# Copyright (c) 2024–2025 Emma Tebibyte <emma@tebibyte.media>
 # SPDX-License-Identifier: FSFAP
 #
 # Copying and distribution of this file, with or without modification, are
 # permitted in any medium without royalty provided the copyright notice and
 # this notice are preserved.  This file is offered as-is, without any warranty.
 
+MM_FILE!=mktemp /tmp/mm.XXXXXX
+
 .PHONY: mm_tests
 mm_tests: mm_args mm_help mm_stderr mm_remaining mm_remaining_options
 
@@ -38,3 +40,18 @@ mm_remaining: $(BIN)/mm
 mm_remaining_options:
 	! $(BIN)/mm -i README COPYING -o - 2>&1 | cut -d: -f2 \
 		| xargs test " -o" =
+
+.PHONY: mm_append
+mm_append:
+	$(BIN)/mm -i /dev/null -o $(MM_FILE)
+	printf 'test\nstring\nmulti-line\n' | $(BIN)/mm -o $(MM_FILE)
+	printf 'new line\n' | $(BIN)/mm -a -o $(MM_FILE)
+	$(BIN)/mm -i $(MM_FILE) | wc -l | xargs test 4 -eq
+
+.PHONY: mm_no_truncate
+mm_no_truncate:
+	$(BIN)/mm -i /dev/null -o $(MM_FILE)
+	printf 'test\nstring\nmulti-line\n' | $(BIN)/mm -o $(MM_FILE)
+	printf 'new line\n' | $(BIN)/mm -t -o $(MM_FILE)
+	$(BIN)/mm -i $(MM_FILE) | wc -l | xargs test 3 -eq
+