diff --git a/README.md b/README.md
index 7d2b4f2..12c618a 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ Use environment variables to select the container engine or override image names
 CONTAINER_ENGINE=podman IMAGE_NAME=my-build-env ./rocky9/build.sh
 ```
 
-The Debian 13 and Rocky Linux 9 images install every library under `/usr/local` by default. Set `PREFIX` to install them elsewhere; the build environment variables (`PATH`, `PKG_CONFIG_PATH`, `CMAKE_PREFIX_PATH`, `CMAKE_INSTALL_PREFIX`, and so on) follow it. Rust is a build tool rather than a library, so it always lives under `/usr/local` and is not affected by `PREFIX`. The Alpine image does not support `PREFIX`.
+The Debian 13 and Rocky Linux 9 images install every library under `/opt/local` by default. Set `PREFIX` to install them elsewhere; the build environment variables (`PATH`, `PKG_CONFIG_PATH`, `CMAKE_PREFIX_PATH`, `CMAKE_INSTALL_PREFIX`, and so on) follow it. Rust is a build tool rather than a library, so it always lives under `/usr/local` and is not affected by `PREFIX`. The Alpine image does not support `PREFIX`.
 
 ```sh
 PREFIX=/opt/stack ./debian13/build.sh
diff --git a/debian13/Dockerfile b/debian13/Dockerfile
index 171cbc2..6a832e5 100644
--- a/debian13/Dockerfile
+++ b/debian13/Dockerfile
@@ -23,11 +23,11 @@ RUN sed -i 's|deb.debian.org|mirrors.ustc.edu.cn|g' /etc/apt/sources.list.d/debi
 
 # Install prefix of every dependency, and the Debian multiarch directory name (`gcc -dumpmachine`)
 # its libraries are installed under; build.sh derives the latter from `uname -m`.
-ARG PREFIX=/usr/local
+ARG PREFIX=/opt/local
 ARG MULTIARCH=x86_64-linux-gnu
 
 # Make the compiler, CMake, pkg-config and the shell find earlier dependencies under a prefix other
-# than the default /usr/local. CMake (>= 3.29) takes its default install prefix from the environment.
+# than the default /opt/local. CMake (>= 3.29) takes its default install prefix from the environment.
 ENV PATH=${PREFIX}/bin:${PATH} \
     C_INCLUDE_PATH=${PREFIX}/include \
     CPLUS_INCLUDE_PATH=${PREFIX}/include \
diff --git a/debian13/build.sh b/debian13/build.sh
index b1cfe23..d9c50bf 100755
--- a/debian13/build.sh
+++ b/debian13/build.sh
@@ -3,7 +3,7 @@
 set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-debian13-source-stack}"
-PREFIX="${PREFIX:-/usr/local}"
+PREFIX="${PREFIX:-/opt/local}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
 SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
 FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"
diff --git a/rocky9/Dockerfile b/rocky9/Dockerfile
index ff35ab6..02fe9c2 100644
--- a/rocky9/Dockerfile
+++ b/rocky9/Dockerfile
@@ -38,10 +38,10 @@ RUN dnf install --assumeyes --enablerepo=crb \
 RUN git config --global url."https://wangguofang.net/github.com/".insteadOf "https://github.com/"
 
 # Install prefix of every dependency. Libraries are installed under ${PREFIX}/lib64.
-ARG PREFIX=/usr/local
+ARG PREFIX=/opt/local
 
 # Make the compiler, CMake, pkg-config and the shell find earlier dependencies under a prefix other
-# than the default /usr/local. CMake (>= 3.29) takes its default install prefix from the environment.
+# than the default /opt/local. CMake (>= 3.29) takes its default install prefix from the environment.
 ENV PATH=${PREFIX}/bin:${PATH} \
     C_INCLUDE_PATH=${PREFIX}/include \
     CPLUS_INCLUDE_PATH=${PREFIX}/include \
diff --git a/rocky9/build.sh b/rocky9/build.sh
index 4e707f8..33f47f0 100755
--- a/rocky9/build.sh
+++ b/rocky9/build.sh
@@ -3,7 +3,7 @@
 set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-rocky9-source-stack}"
-PREFIX="${PREFIX:-/usr/local}"
+PREFIX="${PREFIX:-/opt/local}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
 SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
 FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"