diff --git a/base/grisu/grisu.jl b/base/grisu/grisu.jl
index 14a1ac41347d8..ea36d9a642ae7 100644
--- a/base/grisu/grisu.jl
+++ b/base/grisu/grisu.jl
@@ -26,6 +26,29 @@ include("grisu/bignum.jl")
 
 const BIGNUMS = [Bignums.Bignum(),Bignums.Bignum(),Bignums.Bignum(),Bignums.Bignum()]
 
+"""
+    (len, point, neg) = Grisu.grisu(v::AbstractFloat, mode, requested_digits,
+                buffer=DIGITSs[Threads.threadid()], bignums=BIGNUMS)
+
+Convert the number `v` to decimal using the Grisu algorithm.
+
+The characters are written as bytes to `buffer`, with a terminating NUL byte, and the
+returned tuple contains:
+
+ - `len`: the number of digits written to `buffer` (excluding NUL)
+ - `point`: the location of the radix point relative to the start of the array (e.g. if
+   `point == 3`, then the radix point should be inserted between the 3rd and 4th
+   digit). Note that this can be negative (for very small values), or greater than `len`
+   (for very large values).
+ - `neg`: the signbit of `v` (see [`signbit`](@ref)).
+
+`mode` can be one of:
+ - `Grisu.SHORTEST`: convert to the shortest decimal representation which can be "round-tripped" back to `v`.
+ - `Grisu.FIXED`: round to `requested_digits` digits.
+ - `Grisu.PRECISION`: round to `requested_digits` significant digits.
+
+`bignums` are used internally as part of the correction step.
+"""
 function grisu(v::AbstractFloat,mode,requested_digits,buffer=DIGITSs[Threads.threadid()],bignums=BIGNUMS)
     if signbit(v)
         neg = true