diff --git a/docs/src/data/sensor-humidity.csv b/docs/src/data/sensor-humidity.csv
new file mode 100644
index 000000000..c22ff5d7d
--- /dev/null
+++ b/docs/src/data/sensor-humidity.csv
@@ -0,0 +1,3 @@
+unixTime,minValue,averageValue,maxValue
+1740000000,50.1,50.3,50.5
+1740000120,52.3,52.5,52.7
diff --git a/docs/src/data/sensor-pressure.csv b/docs/src/data/sensor-pressure.csv
new file mode 100644
index 000000000..341104a14
--- /dev/null
+++ b/docs/src/data/sensor-pressure.csv
@@ -0,0 +1,4 @@
+unixTime,minValue,averageValue,maxValue
+1740000000,1012.2,1012.3,1012.4
+1740000060,1011.8,1011.9,1012.0
+1740000120,1011.5,1011.6,1011.7
diff --git a/docs/src/data/sensor-temp.csv b/docs/src/data/sensor-temp.csv
new file mode 100644
index 000000000..a07b02046
--- /dev/null
+++ b/docs/src/data/sensor-temp.csv
@@ -0,0 +1,4 @@
+unixTime,minValue,averageValue,maxValue
+1740000000,37.2,37.4,37.6
+1740000060,37.5,37.6,37.7
+1740000120,37.8,37.9,38.0
diff --git a/docs/src/questions-about-joins.md b/docs/src/questions-about-joins.md
index d7f7e70e2..c620e8190 100644
--- a/docs/src/questions-about-joins.md
+++ b/docs/src/questions-about-joins.md
@@ -277,6 +277,108 @@ id status name task
30 occupied Alice clean
+## Merging several files on a common key when column names collide
+
+The previous example worked painlessly because the non-key column names -- `name` and `status` -- were different in each lookup file. Now suppose you have several files, each containing measurements of a _different_ quantity, but all with the _same_ column names -- say, one file each for temperature, humidity, and pressure, keyed by timestamp:
+
+
+cat data/sensor-temp.csv
+
+
+unixTime,minValue,averageValue,maxValue
+1740000000,37.2,37.4,37.6
+1740000060,37.5,37.6,37.7
+1740000120,37.8,37.9,38.0
+
+
+
+cat data/sensor-humidity.csv
+
+
+unixTime,minValue,averageValue,maxValue
+1740000000,50.1,50.3,50.5
+1740000120,52.3,52.5,52.7
+
+
+
+cat data/sensor-pressure.csv
+
+
+unixTime,minValue,averageValue,maxValue
+1740000000,1012.2,1012.3,1012.4
+1740000060,1011.8,1011.9,1012.0
+1740000120,1011.5,1011.6,1011.7
+
+
+(Note that the humidity file is missing a row for the middle timestamp.)
+
+If we merge these with a `then`-chain of `join` commands, as in the previous section, columns are lost: since every file's non-key columns have the same names, each join step overwrites the values from the step before, and only one file's values survive:
+
+
+mlr --icsv --opprint \
+ join -j unixTime -f data/sensor-temp.csv \
+ then join -j unixTime -f data/sensor-humidity.csv \
+ data/sensor-pressure.csv
+
+
+unixTime minValue averageValue maxValue
+1740000000 1012.2 1012.3 1012.4
+1740000120 1011.5 1011.6 1011.7
+
+
+The fix is `join`'s `--lp` (left-prefix) option, which renames the non-key columns coming from each `-f` file so that nothing collides:
+
+
+mlr --icsv --opprint \
+ join --lp temp: -j unixTime -f data/sensor-temp.csv \
+ then join --lp humidity: -j unixTime -f data/sensor-humidity.csv \
+ data/sensor-pressure.csv
+
+
+unixTime humidity:minValue humidity:averageValue humidity:maxValue temp:minValue temp:averageValue temp:maxValue minValue averageValue maxValue
+1740000000 50.1 50.3 50.5 37.2 37.4 37.6 1012.2 1012.3 1012.4
+1740000120 52.3 52.5 52.7 37.8 37.9 38.0 1011.5 1011.6 1011.7
+
+
+The columns from the file at the end of the command line -- here, the pressure file -- keep their unprefixed names.
+
+If you want just one value column per file, you can also use `join`'s `--lk` option to keep only that column from each `-f` file, then use [cut](reference-verbs.md#cut) and [label](reference-verbs.md#label) to arrange and rename the output columns:
+
+
+mlr --icsv --opprint \
+ join --lp temp: --lk averageValue -j unixTime -f data/sensor-temp.csv \
+ then join --lp humidity: --lk averageValue -j unixTime -f data/sensor-humidity.csv \
+ then cut -o -f unixTime,temp:averageValue,humidity:averageValue,averageValue \
+ then label unixTime,temperature,humidity,pressure \
+ data/sensor-pressure.csv
+
+
+unixTime temperature humidity pressure
+1740000000 37.4 50.3 1012.3
+1740000120 37.9 52.5 1011.6
+
+
+Note that `join` gives inner-join semantics by default, so the timestamp missing from the humidity file has been dropped from the output. (This is also why the `paste` command is not a substitute for `join` here: `paste` matches rows by position, so a row missing from one file shifts that file's remaining values onto the wrong rows.) If you'd rather keep those rows, with empty cells where a file has no data, add `--ul --ur` to each join step, and [unsparsify](reference-verbs.md#unsparsify) afterward:
+
+
+mlr --icsv --opprint \
+ join --ul --ur --lp temp: --lk averageValue -j unixTime -f data/sensor-temp.csv \
+ then join --ul --ur --lp humidity: --lk averageValue -j unixTime -f data/sensor-humidity.csv \
+ then unsparsify \
+ then cut -o -f unixTime,temp:averageValue,humidity:averageValue,averageValue \
+ then label unixTime,temperature,humidity,pressure \
+ then sort -t unixTime \
+ data/sensor-pressure.csv
+
+
+unixTime temperature humidity pressure
+1740000000 37.4 50.3 1012.3
+1740000060 37.6 - 1011.9
+1740000120 37.9 52.5 1011.6
+
+
+The `unsparsify` must come before the `cut`-and-`label` step, since `label` renames columns positionally. The `sort` is there because unpaired records may be emitted out of order relative to paired ones.
+
## How to preprocess the left file of a join?
The left file (the `-f` argument to `join`) is opened by the `join` verb itself, so it doesn't pass through the main record stream: a `then`-chain can preprocess the right file(s), but not the left one.
diff --git a/docs/src/questions-about-joins.md.in b/docs/src/questions-about-joins.md.in
index 368e6bda5..76328a2d7 100644
--- a/docs/src/questions-about-joins.md.in
+++ b/docs/src/questions-about-joins.md.in
@@ -130,6 +130,70 @@ mlr --icsv --opprint join -f multi-join/name-lookup.csv -j id \
multi-join/input.csv
GENMD-EOF
+## Merging several files on a common key when column names collide
+
+The previous example worked painlessly because the non-key column names -- `name` and `status` -- were different in each lookup file. Now suppose you have several files, each containing measurements of a _different_ quantity, but all with the _same_ column names -- say, one file each for temperature, humidity, and pressure, keyed by timestamp:
+
+GENMD-RUN-COMMAND
+cat data/sensor-temp.csv
+GENMD-EOF
+
+GENMD-RUN-COMMAND
+cat data/sensor-humidity.csv
+GENMD-EOF
+
+GENMD-RUN-COMMAND
+cat data/sensor-pressure.csv
+GENMD-EOF
+
+(Note that the humidity file is missing a row for the middle timestamp.)
+
+If we merge these with a `then`-chain of `join` commands, as in the previous section, columns are lost: since every file's non-key columns have the same names, each join step overwrites the values from the step before, and only one file's values survive:
+
+GENMD-RUN-COMMAND
+mlr --icsv --opprint \
+ join -j unixTime -f data/sensor-temp.csv \
+ then join -j unixTime -f data/sensor-humidity.csv \
+ data/sensor-pressure.csv
+GENMD-EOF
+
+The fix is `join`'s `--lp` (left-prefix) option, which renames the non-key columns coming from each `-f` file so that nothing collides:
+
+GENMD-RUN-COMMAND
+mlr --icsv --opprint \
+ join --lp temp: -j unixTime -f data/sensor-temp.csv \
+ then join --lp humidity: -j unixTime -f data/sensor-humidity.csv \
+ data/sensor-pressure.csv
+GENMD-EOF
+
+The columns from the file at the end of the command line -- here, the pressure file -- keep their unprefixed names.
+
+If you want just one value column per file, you can also use `join`'s `--lk` option to keep only that column from each `-f` file, then use [cut](reference-verbs.md#cut) and [label](reference-verbs.md#label) to arrange and rename the output columns:
+
+GENMD-RUN-COMMAND
+mlr --icsv --opprint \
+ join --lp temp: --lk averageValue -j unixTime -f data/sensor-temp.csv \
+ then join --lp humidity: --lk averageValue -j unixTime -f data/sensor-humidity.csv \
+ then cut -o -f unixTime,temp:averageValue,humidity:averageValue,averageValue \
+ then label unixTime,temperature,humidity,pressure \
+ data/sensor-pressure.csv
+GENMD-EOF
+
+Note that `join` gives inner-join semantics by default, so the timestamp missing from the humidity file has been dropped from the output. (This is also why the `paste` command is not a substitute for `join` here: `paste` matches rows by position, so a row missing from one file shifts that file's remaining values onto the wrong rows.) If you'd rather keep those rows, with empty cells where a file has no data, add `--ul --ur` to each join step, and [unsparsify](reference-verbs.md#unsparsify) afterward:
+
+GENMD-RUN-COMMAND
+mlr --icsv --opprint \
+ join --ul --ur --lp temp: --lk averageValue -j unixTime -f data/sensor-temp.csv \
+ then join --ul --ur --lp humidity: --lk averageValue -j unixTime -f data/sensor-humidity.csv \
+ then unsparsify \
+ then cut -o -f unixTime,temp:averageValue,humidity:averageValue,averageValue \
+ then label unixTime,temperature,humidity,pressure \
+ then sort -t unixTime \
+ data/sensor-pressure.csv
+GENMD-EOF
+
+The `unsparsify` must come before the `cut`-and-`label` step, since `label` renames columns positionally. The `sort` is there because unpaired records may be emitted out of order relative to paired ones.
+
## How to preprocess the left file of a join?
The left file (the `-f` argument to `join`) is opened by the `join` verb itself, so it doesn't pass through the main record stream: a `then`-chain can preprocess the right file(s), but not the left one.