diff --git a/Enigma/Rotor.swift b/Enigma/Rotor.swift
index fa6c2a7..1f0d83f 100644
--- a/Enigma/Rotor.swift
+++ b/Enigma/Rotor.swift
@@ -9,18 +9,32 @@
 import Foundation
 
 
-class Rotor {
+protocol Encoder {
+    func encode(c: Character) throws -> Character
+}
+
+
+class Cryptor {
+    enum Error: ErrorType {
+        /** Thrown when encode() encounters a character that is not in the alphabet. */
+        case InvalidCharacter(c: Character)
+    }
+
+    /** Array of all possible characters to encrypt. */
+    static let alphabet: [Character] = Array("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters)
+}
+
+
+/** Rotors are Cryptors that have a position, which offsets the alphabet from the series and changes which character is substituted for a given input. */
+class Rotor: Cryptor, Encoder {
     enum Error: ErrorType {
         /** Thrown when the initializer is given an invalid series. */
         case InvalidSeries
-        /** Thrown when encode() encounters a character that is not in the alphabet. */
-        case InvalidCharacter
     }
 
-    static let alphabet: [Character] = Array("ABCDEFGHIJKLMNOPQRSTUVWXYZ".characters)
-
     /** The position of first letter in `series` in the `alphabet`. */
-    var position: Int
+    var position: Int = 0
+
     /** The series of characters that this rotor cycles through. */
     let series: [Character]!
 
@@ -29,12 +43,11 @@ class Rotor {
     }
 
     init(series: [Character]) throws {
-        self.position = 0
-        guard series.count == Rotor.alphabet.count else {
-            self.series = nil
+        self.series = series
+        super.init()
+        guard series.count == Cryptor.alphabet.count else {
             throw Error.InvalidSeries
         }
-        self.series = series
     }
 
     func advance(count: Int = 1) {
@@ -44,8 +57,64 @@ class Rotor {
     func encode(c: Character) throws -> Character {
         let offset: Int! = Rotor.alphabet.indexOf(c)
         guard offset != nil else {
-            throw Error.InvalidCharacter
+            throw Error.InvalidCharacter(c: c)
         }
         return series[(offset + position) % series.count]
     }
+}
+
+
+/** A Plugboard is a Cryptor that substitutes one character for another based on a set of pairs. A pair of characters is mutually exclusive of other pairs; that is, a character can only belong to one pair. Furthermore, the Plugboard always trades one character for the same character and vice versa. */
+class Plugboard: Cryptor, Encoder {
+    enum Error: ErrorType {
+        case CharacterInMultiplePairs(c: Character)
+        case PairContainsSameCharacter(c: Character)
+    }
+
+    var pairs: [(Character, Character)]!
+
+    init(pairs: [(Character, Character)] = []) throws {
+        super.init()
+        try validatePairs(pairs)
+        self.pairs = pairs
+    }
+
+    func validatePairs(pairs: [(Character, Character)]) throws {
+        let alphabetSet = Set(Cryptor.alphabet)
+        var charactersSeen = Set<Character>()
+        for pair in pairs {
+            if !alphabetSet.contains(pair.0) {
+                throw Cryptor.Error.InvalidCharacter(c: pair.0)
+            }
+            if  !alphabetSet.contains(pair.1) {
+                throw Cryptor.Error.InvalidCharacter(c: pair.1)
+            }
+            if pair.0 == pair.1 {
+                throw Error.PairContainsSameCharacter(c: pair.0)
+            }
+            if charactersSeen.contains(pair.0) {
+                throw Error.CharacterInMultiplePairs(c: pair.0)
+            } else {
+                charactersSeen.insert(pair.0)
+            }
+            if charactersSeen.contains(pair.1) {
+                throw Error.CharacterInMultiplePairs(c: pair.1)
+            } else {
+                charactersSeen.insert(pair.1)
+            }
+        }
+    }
+
+    func encode(c: Character) throws -> Character {
+        for pair in pairs {
+            if c == pair.0 {
+                return pair.1
+            }
+            if c == pair.1 {
+                return pair.0
+            }
+        }
+        // If no pair exists, just return the character itself.
+        return c
+    }
 }
\ No newline at end of file
diff --git a/EnigmaTests/EnigmaTests.swift b/EnigmaTests/EnigmaTests.swift
index b34248d..faf5c27 100644
--- a/EnigmaTests/EnigmaTests.swift
+++ b/EnigmaTests/EnigmaTests.swift
@@ -9,8 +9,9 @@
 import XCTest
 @testable import Enigma
 
+let alphaSeries = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
 class RotorTests: XCTestCase {
-    let alphaSeries = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
     let rotorSeries = "EKMFLGDQVZNTOWYHXUSPAIBRCJ"
 
     func testThatUnadvancedSubstitutionWorks() {
@@ -28,4 +29,20 @@ class RotorTests: XCTestCase {
             XCTAssertEqual(try! rotor.encode(plainCharacter), cipherCharacter)
         }
     }
+}
+
+
+class PlugboardTests: XCTestCase {
+    func testThatEmptyPlugboardPassesThroughAllCharacters() {
+        let plugboard = try! Enigma.Plugboard()
+        for c in alphaSeries.characters {
+            XCTAssertEqual(try! plugboard.encode(c), c)
+        }
+    }
+
+    func testThatPlugboardPairsAreBidirectional() {
+        let plugboard = try! Enigma.Plugboard(pairs: [("A", "H")])
+        XCTAssertEqual(try! plugboard.encode("A"), "H")
+        XCTAssertEqual(try! plugboard.encode("H"), "A")
+    }
 }
\ No newline at end of file