diff --git a/base/abstractdict.jl b/base/abstractdict.jl
index b0f5784daf453..7e5e88cf7dbed 100644
--- a/base/abstractdict.jl
+++ b/base/abstractdict.jl
@@ -69,6 +69,16 @@ in(k, v::KeySet) = get(v.dict, k, secret_table_token) !== secret_table_token
 
 For an iterator or collection that has keys and values (e.g. arrays and dictionaries),
 return an iterator over the keys.
+
+# Implementation
+
+It is highly recommended that each `collection` that implements `keys`
+to satisfy the following relationship between [`values`](@ref) and
+[`getindex`](@ref):
+
+```julia
+isequal(collect(values(collection)), [collection[k] for k in keys(collection)])
+```
 """
 function keys end
 
@@ -130,6 +140,18 @@ values(a::AbstractDict) = ValueIterator(a)
 Return an iterator over `key => value` pairs for any
 collection that maps a set of keys to a set of values.
 This includes arrays, where the keys are the array indices.
+
+# Implementation
+
+It is highly recommended that each implementation of `pairs` satisfies
+the following relationships:
+
+```julia
+isequal(map(first, paris(collection)), keys(collection))
+isequal(map(last, paris(collection)), values(collection))
+```
+
+The fallback generic implementation satisfy this invariance.
 """
 pairs(collection) = Generator(=>, keys(collection), values(collection))
 
diff --git a/base/array.jl b/base/array.jl
index 8954f3e5b9d23..eb3ff6a8672c4 100644
--- a/base/array.jl
+++ b/base/array.jl
@@ -804,6 +804,11 @@ Dict{String,Int64} with 2 entries:
 julia> getindex(A, "a")
 1
 ```
+
+# Implementation
+
+Implementation of `getindex` should satisfy the invariance explained
+in [`keys`](@ref).
 """
 function getindex end
 
diff --git a/base/essentials.jl b/base/essentials.jl
index e5870bdb9b893..76a8831b0a5bf 100644
--- a/base/essentials.jl
+++ b/base/essentials.jl
@@ -758,6 +758,11 @@ julia> values([2])
 1-element Array{Int64,1}:
  2
 ```
+
+# Implementation
+
+Implementation of `values` should satisfy the invariance explained in
+[`keys`](@ref).
 """
 values(itr) = itr