diff --git a/day9/day9.jl b/day9/day9.jl
index 449d995..e39ce5c 100644
--- a/day9/day9.jl
+++ b/day9/day9.jl
@@ -7,7 +7,7 @@ struct Move
   count :: Int
 end
 
-mutable struct State
+mutable struct KnotPair
   head_x :: Int
   head_y :: Int
   tail_x :: Int
@@ -19,6 +19,14 @@ function parse_move(line)
   return Move(parts[1][1], parse(Int, parts[2]))
 end
 
+
+function state_changed(old_state, new_state)
+  return (new_state.head_x != old_state.head_x
+          || new_state.head_y != old_state.head_y
+          || new_state.tail_x != old_state.tail_x
+          || new_state.tail_y != old_state.tail_y)
+end
+
 function read_moves(io)
   return [parse_move(line) for line in readlines(io)]
 end
@@ -32,6 +40,8 @@ function apply_move(state, direction)
     state.head_x -= 1
   elseif direction == 'R'
     state.head_x += 1
+  elseif direction != 'O'
+    throw(DomainError("Unknown direction"))
   end
   x_diff = state.head_x - state.tail_x
   y_diff = state.head_y - state.tail_y
@@ -44,20 +54,36 @@ function apply_move(state, direction)
   elseif abs(y_diff) <= 1
     state.tail_y += y_diff
     state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
+  elseif abs(x_diff) == 2 && abs(y_diff) == 2
+    # possible with more than 2 knots
+    state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
+    state.tail_y += sign(y_diff) * (abs(y_diff) - 1)
   else
-    throw(DomainError("invalid state"))
+    throw(DomainError("invalid state " * string(state)))
   end 
   return state
 end
 
-function get_visited(moves)
+function get_visited(moves; npairs::Int=1)
   visited = Set{NTuple{2, Int}}()
-  state = State(0, 0, 0, 0)
-  push!(visited, (state.tail_x, state.tail_y))
+  states = [KnotPair(0, 0, 0, 0) for _ in 1:npairs]
+  push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
   for move in moves
-    for _ in 1:move.count
-      state = apply_move(state, move.direction)
-      push!(visited, (state.tail_x, state.tail_y))
+    for m in 1:move.count
+      # println(move, " ", m)
+      # print(" 1 ", states[1], " -> ")
+      states[1] = apply_move(states[1], move.direction)
+      # println(states[1])
+      for i in 2:npairs
+        states[i].head_x = states[i-1].tail_x
+        states[i].head_y = states[i-1].tail_y
+        # old_state = deepcopy(states[i])
+        states[i] = apply_move(states[i], 'O')
+        # if state_changed(states[i], old_state)
+        #   println(" ", i, " ", old_state, " -> ", states[i])
+        # end
+      end
+      push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
     end
   end
   return visited
@@ -76,10 +102,14 @@ function visited_map(visited)
 end
 
 function test()
-  @testset "visited count" verbose=true begin
+  @testset "visited count 2 knot" verbose=true begin
     @test length(get_visited(read_moves("example.txt"))) == 13
     @test length(get_visited(read_moves("input.txt"))) == 6037
   end
+  @testset "visited count 10 knot" verbose=true begin
+    @test length(get_visited(read_moves("example.txt"), npairs=9)) == 1
+    @test length(get_visited(read_moves("input.txt"), npairs=9)) == 2485
+  end
 end
 
 function main()
@@ -95,6 +125,13 @@ function main()
     println("count  : ", length(visited))
     display(visited_map(visited))
     println()
+
+    visited10 = get_visited(moves, npairs=9)
+    println("visited10: ", visited10)
+    println("count10  : ", length(visited10))
+    display(visited_map(visited10))
+    println()
+
   end
 end
 
diff --git a/day9/example2.txt b/day9/example2.txt
new file mode 100644
index 0000000..60bd43b
--- /dev/null
+++ b/day9/example2.txt
@@ -0,0 +1,8 @@
+R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20