--- a/src/Grids/grid.jl	Thu Nov 23 16:08:47 2023 +0100
+++ b/src/Grids/grid.jl	Fri Dec 01 11:52:26 2023 +0100
@@ -74,6 +74,18 @@
 # TBD: Can we implement a version here that accepts multiple ids and grouped boundaries? Maybe we need multiblock stuff?
 
 """
+    boundary_indices(g::Grid, id::BoundaryIdentifier)
+
+A collection of indices corresponding to the boundary with given id. For grids
+with Cartesian indexing these collections will be tuples with elements of type
+``Union{Int,Colon}``.
+
+When implementing this method it is expected that the returned collection can
+be used to index grid functions to obtain grid functions on the boundary grid.
+"""
+function boundary_indices end
+
+"""
     eval_on(g::Grid, f)
 
 Lazy evaluation of `f` on the grid. `f` can either be on the form `f(x,y,...)`
@@ -100,3 +112,5 @@
 
 _ncomponents(::Type{<:Number}) = 1
 _ncomponents(T::Type{<:SVector}) = length(T)
+
+