diff --git a/R/round.R b/R/round.R
index 7b4809d..82be1ef 100644
--- a/R/round.R
+++ b/R/round.R
@@ -114,6 +114,10 @@ round_up_from <- function(x, digits = 0L, threshold, symmetric = FALSE) {
 #' @rdname rounding-common
 #' @export
 
+# Note that this implementation is slightly different from the formula for
+# rounding down in the "Rounding in detail" article. However, this is only for
+# performance reasons, and the results are equivalent.
+
 round_down_from <- function(x, digits = 0L, threshold, symmetric = FALSE) {
 
   p10 <- 10 ^ digits
diff --git a/vignettes/rounding-in-detail.Rmd b/vignettes/rounding-in-detail.Rmd
index a04962f..9f057e4 100644
--- a/vignettes/rounding-in-detail.Rmd
+++ b/vignettes/rounding-in-detail.Rmd
@@ -94,10 +94,10 @@ $$
 
 where $x$ is the number to be rounded, $d$ is the number of decimal places to which $x$ should be rounded, and $t$ is the threshold for rounding up (e.g., $t = 5$ for rounding up from 5). Note that $\lfloor n \rfloor$ floors a number $n$, and $\lceil n \rceil$ ceils it.
 
-Rounding down works accordingly:
+Rounding down works accordingly. Note that $+$ and $-$ are reversed here:
 
 $$
-\frac{\lceil x(10^d) -1 - \frac{t}{10} \rceil}{10^d}
+\frac{\lceil x(10^d) - 1 + \frac{t}{10} \rceil}{10^d}
 $$
 
 ### To even (base R)