diff --git a/DESCRIPTION b/DESCRIPTION
index 1fc9fc23..141a9d1d 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: piar
 Title: Price Index Aggregation
-Version: 0.8.3.9003
+Version: 0.8.3.9004
 Authors@R: c(
     person("Steve", "Martin", role = c("aut", "cre", "cph"),
            email = "marberts@protonmail.com",
diff --git a/NAMESPACE b/NAMESPACE
index 8e2ba940..22a0154e 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -93,6 +93,7 @@ export(as_index)
 export(carry_backward)
 export(carry_forward)
 export(chain)
+export(combine_classifications)
 export(contrib)
 export(contrib2DF)
 export(elemental_index)
diff --git a/NEWS.md b/NEWS.md
index e143d0d4..c5e2ba64 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -26,6 +26,11 @@ in `elementary_index()`.
 
 - The `margin` argument in `split(index)` has been renamed to `along`.
 
+## Improvements
+
+- Added a new function `combine_classifications()` to more easily build an
+aggregation structure that spans multiple hierarchical classifications.
+
 ## Bug fixes
 
 - Setting `contrib = FALSE` in `aggregate(index)` always returns an index with
diff --git a/R/combine_classifications.R b/R/combine_classifications.R
new file mode 100644
index 00000000..3cbf785d
--- /dev/null
+++ b/R/combine_classifications.R
@@ -0,0 +1,42 @@
+#' Combine hierarchical classifications
+#'
+#' Combine hierarchical classifications by stacking one classification after
+#' another.
+#'
+#' @param x,y A list with a entry for each level in classification giving the
+#'   "digits" that represent each level in the hierarchy, as made
+#'   by [expand_classification()] or [split_classification()].
+#' @param sep A character used to separate `x` and `y`. The
+#'   default separates levels across classifications by `"."`.
+#' @returns
+#' A list with a entry for each level in the combined classification.
+#'
+#' @examples
+#' # Combine an unbalanced industry classification with a balanced
+#' # geographic classification
+#'
+#' industry <- c("111", "112", "12")
+#' region <- c("11", "21", "22")
+#'
+#' combine_classifications(
+#'   expand_classification(industry, pad = "0"),
+#'   expand_classification(region)
+#' )
+#' @export
+combine_classifications <- function(x, y, sep = ".") {
+  x <- lapply(x, as.character)
+  lx <- lengths(x)
+  if (length(lx) == 0L) {
+    return(y)
+  }
+  y <- lapply(y, as.character)
+  ly <- lengths(y)
+  if (length(ly) == 0L) {
+    return(x)
+  }
+  if (any(lx != lx[1L]) || any(ly != lx[1L])) {
+    stop("each component of 'x' and 'y' must have the same number of elements")
+  }
+
+  c(x, lapply(y, \(y) paste(last(x), y, sep = sep)))
+}
diff --git a/R/expand_classification.R b/R/expand_classification.R
index c3d28437..df53569f 100644
--- a/R/expand_classification.R
+++ b/R/expand_classification.R
@@ -15,7 +15,7 @@
 #'   level of the classification, ordered so that moving down the list goes down
 #'   the hierarchy (as made by `expand_classification()`).
 #' @param sep A character used to combine codes/labels across elements of `...`.
-#'   The default uses ':'.
+#'   The default uses `":"`.
 #' @param pad A string used to pad the shorter labels for an unbalanced
 #'   classification. The default pads with NA.
 #'
@@ -32,6 +32,9 @@
 #' [split_classification()] to expand a classification by splitting along
 #' a delimiter.
 #'
+#' [combine_classifications()] for combining multiple hierarchical
+#' classifications.
+#'
 #' `csh_from_digits()` in the \pkg{accumulate} package for different handling
 #' of unbalanced classifications.
 #'
diff --git a/R/extract.piar_index.R b/R/extract.piar_index.R
index 925a2ffc..9932fe54 100644
--- a/R/extract.piar_index.R
+++ b/R/extract.piar_index.R
@@ -1,4 +1,4 @@
-#' Extract and replace index values
+#' Extract index values
 #'
 #' Methods to extract and replace index values like a matrix.
 #'
diff --git a/R/split_classification.R b/R/split_classification.R
index c40dcc2c..45233596 100644
--- a/R/split_classification.R
+++ b/R/split_classification.R
@@ -10,7 +10,7 @@
 #'   See [strsplit()].
 #' @param ... Additional argument to pass to [strsplit()].
 #' @param sep A character used to delineate levels in `x` in the result. The
-#'   default separates levels by '.'.
+#'   default separates levels by `"."`.
 #' @param pad A string used to pad the shorter labels for an unbalanced
 #'   classification. The default pads with NA.
 #'
@@ -24,6 +24,9 @@
 #' [expand_classification()] to expand a classification by the width of the
 #' levels.
 #'
+#' [combine_classifications()] for combining multiple hierarchical
+#' classifications.
+#'
 #' @examples
 #' #' # A simple classification structure
 #' #            1
diff --git a/R/window.piar_index.R b/R/window.piar_index.R
index bec6639d..d6c3f2fe 100644
--- a/R/window.piar_index.R
+++ b/R/window.piar_index.R
@@ -1,4 +1,4 @@
-#' Index window
+#' Window a price index
 #'
 #' Extract and replace index values over a window of time periods.
 #'
diff --git a/README.Rmd b/README.Rmd
index 3f66fc5a..acf5cd6c 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -116,8 +116,6 @@ guidelines for this project for more information.
 
 Balk, B. M. (2008). *Price and Quantity Index Numbers*. Cambridge University Press.
 
-Chiru, R., Huang, N., Lequain, M. Smith, P., and Wright, A. (2015). *The Canadian Consumer Price Index Reference Paper*, Statistics Canada catalogue 62-553-X. Statistics Canada.
-
 ILO, IMF, UNECE, OECD, and World Bank. (2004). *Producer Price Index Manual: Theory and Practice*. International Monetary Fund.
 
 IMF, ILO, Eurostat, UNECE, OECD, and World Bank. (2020). *Consumer Price Index Manual: Concepts and Methods*. International Monetary Fund.
diff --git a/README.md b/README.md
index 919c841b..94c48f22 100644
--- a/README.md
+++ b/README.md
@@ -106,12 +106,12 @@ And an aggregation structure.
 # industrial classifications
 
 ms_weights
-#>   business classification weight level1 level2
-#> 1       B1             11    553      1     11
-#> 2       B2             11    646      1     11
-#> 3       B3             11    312      1     11
-#> 4       B4             12    622      1     12
-#> 5       B5             12    330      1     12
+#>   business classification weight
+#> 1       B1             11    553
+#> 2       B2             11    646
+#> 3       B3             11    312
+#> 4       B4             12    622
+#> 5       B5             12    330
 
 ms_weights[c("level1", "level2")] <-
   expand_classification(ms_weights$classification)
@@ -165,10 +165,6 @@ the contribution guidelines for this project for more information.
 Balk, B. M. (2008). *Price and Quantity Index Numbers*. Cambridge
 University Press.
 
-Chiru, R., Huang, N., Lequain, M. Smith, P., and Wright, A. (2015). *The
-Canadian Consumer Price Index Reference Paper*, Statistics Canada
-catalogue 62-553-X. Statistics Canada.
-
 ILO, IMF, UNECE, OECD, and World Bank. (2004). *Producer Price Index
 Manual: Theory and Practice*. International Monetary Fund.
 
diff --git a/_pkgdown.yml b/_pkgdown.yml
index e4d3dcf9..ea695c77 100644
--- a/_pkgdown.yml
+++ b/_pkgdown.yml
@@ -67,6 +67,7 @@ reference:
   - aggregation_structure
   - expand_classification
   - split_classification
+  - combine_classifications
   - as_aggregation_structure
   - as.matrix.piar_aggregation_structure
   - weights.piar_aggregation_structure
diff --git a/man/combine_classifications.Rd b/man/combine_classifications.Rd
new file mode 100644
index 00000000..630fc620
--- /dev/null
+++ b/man/combine_classifications.Rd
@@ -0,0 +1,35 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/combine_classifications.R
+\name{combine_classifications}
+\alias{combine_classifications}
+\title{Combine hierarchical classifications}
+\usage{
+combine_classifications(x, y, sep = ".")
+}
+\arguments{
+\item{x, y}{A list with a entry for each level in classification giving the
+"digits" that represent each level in the hierarchy, as made
+by \code{\link[=expand_classification]{expand_classification()}} or \code{\link[=split_classification]{split_classification()}}.}
+
+\item{sep}{A character used to separate \code{x} and \code{y}. The
+default separates levels across classifications by \code{"."}.}
+}
+\value{
+A list with a entry for each level in the combined classification.
+}
+\description{
+Combine hierarchical classifications by stacking one classification after
+another.
+}
+\examples{
+# Combine an unbalanced industry classification with a balanced
+# geographic classification
+
+industry <- c("111", "112", "12")
+region <- c("11", "21", "22")
+
+combine_classifications(
+  expand_classification(industry, pad = "0"),
+  expand_classification(region)
+)
+}
diff --git a/man/expand_classification.Rd b/man/expand_classification.Rd
index 47bb8f19..3c10aae3 100644
--- a/man/expand_classification.Rd
+++ b/man/expand_classification.Rd
@@ -27,7 +27,7 @@ level of the classification, ordered so that moving down the list goes down
 the hierarchy (as made by \code{expand_classification()}).}
 
 \item{sep}{A character used to combine codes/labels across elements of \code{...}.
-The default uses ':'.}
+The default uses \code{":"}.}
 }
 \value{
 \code{expand_classification()} returns a list with a entry for each level
@@ -74,6 +74,9 @@ expand_classification(c("01.1.1", "01.1.2", "01.2.1"), width = 2)
 \code{\link[=split_classification]{split_classification()}} to expand a classification by splitting along
 a delimiter.
 
+\code{\link[=combine_classifications]{combine_classifications()}} for combining multiple hierarchical
+classifications.
+
 \code{csh_from_digits()} in the \pkg{accumulate} package for different handling
 of unbalanced classifications.
 }
diff --git a/man/split_classification.Rd b/man/split_classification.Rd
index 9a4d0487..d0c26cb0 100644
--- a/man/split_classification.Rd
+++ b/man/split_classification.Rd
@@ -17,7 +17,7 @@ See \code{\link[=strsplit]{strsplit()}}.}
 \item{...}{Additional argument to pass to \code{\link[=strsplit]{strsplit()}}.}
 
 \item{sep}{A character used to delineate levels in \code{x} in the result. The
-default separates levels by '.'.}
+default separates levels by \code{"."}.}
 
 \item{pad}{A string used to pad the shorter labels for an unbalanced
 classification. The default pads with NA.}
@@ -50,4 +50,7 @@ split_classification(c("01.1.1", "01.1.2", "01.2.1"), ".", fixed = TRUE)
 
 \code{\link[=expand_classification]{expand_classification()}} to expand a classification by the width of the
 levels.
+
+\code{\link[=combine_classifications]{combine_classifications()}} for combining multiple hierarchical
+classifications.
 }
diff --git a/man/sub-.piar_index.Rd b/man/sub-.piar_index.Rd
index db8d78df..5d141f47 100644
--- a/man/sub-.piar_index.Rd
+++ b/man/sub-.piar_index.Rd
@@ -3,7 +3,7 @@
 \name{[.piar_index}
 \alias{[.piar_index}
 \alias{[<-.piar_index}
-\title{Extract and replace index values}
+\title{Extract index values}
 \usage{
 \method{[}{piar_index}(x, i, j, ...)
 
diff --git a/man/window.piar_index.Rd b/man/window.piar_index.Rd
index 4f86b981..569b48e0 100644
--- a/man/window.piar_index.Rd
+++ b/man/window.piar_index.Rd
@@ -3,7 +3,7 @@
 \name{window.piar_index}
 \alias{window.piar_index}
 \alias{window<-.piar_index}
-\title{Index window}
+\title{Window a price index}
 \usage{
 \method{window}{piar_index}(x, start = NULL, end = NULL, ...)
 
diff --git a/tests/Examples/piar-Ex.Rout.save b/tests/Examples/piar-Ex.Rout.save
index 0b226955..a4354e3c 100644
--- a/tests/Examples/piar-Ex.Rout.save
+++ b/tests/Examples/piar-Ex.Rout.save
@@ -1,5 +1,5 @@
 
-R version 4.5.0 (2025-04-11) -- "How About a Twenty-Six"
+R version 4.5.2 (2025-10-31) -- "[Not] Part in a Rumble"
 Copyright (C) 2025 The R Foundation for Statistical Computing
 Platform: x86_64-pc-linux-gnu
 
@@ -370,6 +370,46 @@ levels         1 2 3
      2 0.2000000 1 8
      3 0.1666667 1 9
 > 
+> 
+> 
+> 
+> cleanEx()
+> nameEx("combine_classifications")
+> ### * combine_classifications
+> 
+> flush(stderr()); flush(stdout())
+> 
+> ### Name: combine_classifications
+> ### Title: Combine hierarchical classifications
+> ### Aliases: combine_classifications
+> 
+> ### ** Examples
+> 
+> # Combine an unbalanced industry classification with a balanced
+> # geographic classification
+> 
+> industry <- c("111", "112", "12")
+> region <- c("11", "21", "22")
+> 
+> combine_classifications(
++   expand_classification(industry, pad = "0"),
++   expand_classification(region)
++ )
+[[1]]
+[1] "1" "1" "1"
+
+[[2]]
+[1] "11" "11" "12"
+
+[[3]]
+[1] "111" "112" "120"
+
+[[4]]
+[1] "111.1" "112.2" "120.2"
+
+[[5]]
+[1] "111.11" "112.21" "120.22"
+
 > 
 > 
 > 
@@ -405,7 +445,7 @@ levels         1 2 3
 > 
 > contrib(index)
        time
-product        1         2
+product         1         2
     a.1 0.0000000 0.5081686
     a.2 0.2440169 0.6442213
     b.1 0.3905243 2.0513858
@@ -945,7 +985,7 @@ levels 1 2 3 4 5 6
 > flush(stderr()); flush(stdout())
 > 
 > ### Name: [.piar_index
-> ### Title: Extract and replace index values
+> ### Title: Extract index values
 > ### Aliases: [.piar_index [<-.piar_index
 > 
 > ### ** Examples
@@ -1094,7 +1134,7 @@ $ea
 > flush(stderr()); flush(stdout())
 > 
 > ### Name: window.piar_index
-> ### Title: Index window
+> ### Title: Window a price index
 > ### Aliases: window.piar_index window<-.piar_index
 > 
 > ### ** Examples
@@ -1126,7 +1166,7 @@ levels 1 2 3
 > cleanEx()
 > options(digits = 7L)
 > base::cat("Time elapsed: ", proc.time() - base::get("ptime", pos = 'CheckExEnv'),"\n")
-Time elapsed:  0.126 0.008 0.134 0 0 
+Time elapsed:  0.277 0.027 0.316 0 0 
 > grDevices::dev.off()
 null device 
           1 
diff --git a/tests/testthat/test-combine_classifications.R b/tests/testthat/test-combine_classifications.R
new file mode 100644
index 00000000..79b22ace
--- /dev/null
+++ b/tests/testthat/test-combine_classifications.R
@@ -0,0 +1,26 @@
+test_that("combining classifications works", {
+  x <- c("111", "121", "122")
+  y <- c("01", "02", "01")
+  expect_identical(
+    combine_classifications(
+      expand_classification(x),
+      expand_classification(y)
+    ),
+    expand_classification(paste(x, y, sep = "."), c(1, 1, 1, 2, 1))
+  )
+})
+
+test_that("corner cases work", {
+  x <- list(c("11", "12"), c("111", "121"))
+  expect_identical(combine_classifications(x, list()), x)
+  expect_identical(combine_classifications(list(), x), x)
+})
+
+test_that("combining classifications errors with unequal length", {
+  expect_error(
+    combine_classifications(
+      list(c("1", "1"), c("11", "12")),
+      list(c("01", "02"), c("011"))
+    )
+  )
+})