diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 44f629ee4..1d2903758 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -614,6 +614,8 @@
 		4BC751B21D157E61006C31D9 /* 6522Tests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4BC751B11D157E61006C31D9 /* 6522Tests.swift */; };
 		4BC76E691C98E31700E6EF73 /* FIRFilter.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC76E671C98E31700E6EF73 /* FIRFilter.cpp */; };
 		4BC76E6B1C98F43700E6EF73 /* Accelerate.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 4BC76E6A1C98F43700E6EF73 /* Accelerate.framework */; };
+		4BC891AD20F6EAB300EDE5B3 /* Rectangle.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC891AB20F6EAB300EDE5B3 /* Rectangle.cpp */; };
+		4BC891AE20F6EAB300EDE5B3 /* Rectangle.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC891AB20F6EAB300EDE5B3 /* Rectangle.cpp */; };
 		4BC91B831D1F160E00884B76 /* CommodoreTAP.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC91B811D1F160E00884B76 /* CommodoreTAP.cpp */; };
 		4BC9DF451D044FCA00F44158 /* ROMImages in Resources */ = {isa = PBXBuildFile; fileRef = 4BC9DF441D044FCA00F44158 /* ROMImages */; };
 		4BC9DF4F1D04691600F44158 /* 6560.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC9DF4D1D04691600F44158 /* 6560.cpp */; };
@@ -1356,6 +1358,8 @@
 		4BC76E671C98E31700E6EF73 /* FIRFilter.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = FIRFilter.cpp; sourceTree = "<group>"; };
 		4BC76E681C98E31700E6EF73 /* FIRFilter.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = FIRFilter.hpp; sourceTree = "<group>"; };
 		4BC76E6A1C98F43700E6EF73 /* Accelerate.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Accelerate.framework; path = System/Library/Frameworks/Accelerate.framework; sourceTree = SDKROOT; };
+		4BC891AB20F6EAB300EDE5B3 /* Rectangle.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = Rectangle.cpp; sourceTree = "<group>"; };
+		4BC891AC20F6EAB300EDE5B3 /* Rectangle.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Rectangle.hpp; sourceTree = "<group>"; };
 		4BC91B811D1F160E00884B76 /* CommodoreTAP.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = CommodoreTAP.cpp; sourceTree = "<group>"; };
 		4BC91B821D1F160E00884B76 /* CommodoreTAP.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = CommodoreTAP.hpp; sourceTree = "<group>"; };
 		4BC9DF441D044FCA00F44158 /* ROMImages */ = {isa = PBXFileReference; lastKnownFileType = folder; name = ROMImages; path = ../../../../ROMImages; sourceTree = "<group>"; };
@@ -2896,6 +2900,7 @@
 			children = (
 				4B5073051DDD3B9400C48FBD /* ArrayBuilder.cpp */,
 				4BBF990A1C8FBA6F0075DAFB /* CRTOpenGL.cpp */,
+				4BC891AB20F6EAB300EDE5B3 /* Rectangle.cpp */,
 				4BBF99081C8FBA6F0075DAFB /* TextureBuilder.cpp */,
 				4BBF99121C8FBA6F0075DAFB /* TextureTarget.cpp */,
 				4B5073061DDD3B9400C48FBD /* ArrayBuilder.hpp */,
@@ -2903,6 +2908,7 @@
 				4BBF990B1C8FBA6F0075DAFB /* CRTOpenGL.hpp */,
 				4BBF990E1C8FBA6F0075DAFB /* Flywheel.hpp */,
 				4BBF990F1C8FBA6F0075DAFB /* OpenGL.hpp */,
+				4BC891AC20F6EAB300EDE5B3 /* Rectangle.hpp */,
 				4BBF99091C8FBA6F0075DAFB /* TextureBuilder.hpp */,
 				4BBF99131C8FBA6F0075DAFB /* TextureTarget.hpp */,
 				4BC3B74C1CD194CC00F86E85 /* Shaders */,
@@ -3614,6 +3620,7 @@
 				4B055AD81FAE9B180060FFFF /* Video.cpp in Sources */,
 				4B89452F201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
 				4B894531201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
+				4BC891AE20F6EAB300EDE5B3 /* Rectangle.cpp in Sources */,
 				4B894539201967B4007DE474 /* Tape.cpp in Sources */,
 				4B055AE51FAE9B6F0060FFFF /* IntermediateShader.cpp in Sources */,
 				4B15A9FD208249BB005E6C8D /* StaticAnalyser.cpp in Sources */,
@@ -3840,6 +3847,7 @@
 				4BD3A30B1EE755C800B5B501 /* Video.cpp in Sources */,
 				4BBF99141C8FBA6F0075DAFB /* TextureBuilder.cpp in Sources */,
 				4B5FADBA1DE3151600AEC565 /* FileHolder.cpp in Sources */,
+				4BC891AD20F6EAB300EDE5B3 /* Rectangle.cpp in Sources */,
 				4B643F3A1D77AD1900D431D6 /* CSStaticAnalyser.mm in Sources */,
 				4B1497881EE4A1DA00CE2596 /* ZX80O81P.cpp in Sources */,
 				4B894520201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
diff --git a/Outputs/CRT/Internals/Rectangle.cpp b/Outputs/CRT/Internals/Rectangle.cpp
new file mode 100644
index 000000000..6600ef5bd
--- /dev/null
+++ b/Outputs/CRT/Internals/Rectangle.cpp
@@ -0,0 +1,74 @@
+//
+//  Rectangle.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 11/07/2018.
+//  Copyright © 2018 Thomas Harte. All rights reserved.
+//
+
+#include "Rectangle.hpp"
+
+using namespace OpenGL;
+
+Rectangle::Rectangle(float x, float y, float width, float height):
+ 	pixel_shader_(
+		"#version 150\n"
+
+		"in vec2 position;"
+
+		"void main(void)"
+		"{"
+			"gl_Position = vec4(position, 0.0, 1.0);"
+		"}",
+
+		"#version 150\n"
+
+		"uniform vec4 colour;"
+		"out vec4 fragColour;"
+
+		"void main(void)"
+		"{"
+			"fragColour = colour;"
+		"}"
+	){
+	pixel_shader_.bind();
+
+	glGenVertexArrays(1, &drawing_vertex_array_);
+	glGenBuffers(1, &drawing_array_buffer_);
+
+	glBindVertexArray(drawing_vertex_array_);
+	glBindBuffer(GL_ARRAY_BUFFER, drawing_array_buffer_);
+
+	GLint position_attribute = pixel_shader_.get_attrib_location("position");
+	glEnableVertexAttribArray(static_cast<GLuint>(position_attribute));
+
+	glVertexAttribPointer(
+		(GLuint)position_attribute,
+		2,
+		GL_FLOAT,
+		GL_FALSE,
+		2 * sizeof(GLfloat),
+		(void *)0);
+
+	colour_uniform_ = pixel_shader_.get_uniform_location("colour");
+
+	float buffer[4*2];
+
+	// Store positions.
+	buffer[0] = x;			buffer[1] = y;
+	buffer[2] = x;			buffer[3] = y + height;
+	buffer[4] = x + width;	buffer[5] = y;
+	buffer[6] = x + width;	buffer[7] = y + height;
+
+	// Upload buffer.
+	glBindBuffer(GL_ARRAY_BUFFER, drawing_array_buffer_);
+	glBufferData(GL_ARRAY_BUFFER, sizeof(buffer), buffer, GL_STATIC_DRAW);
+}
+
+void Rectangle::draw(float red, float green, float blue) {
+	pixel_shader_.bind();
+	glUniform4f(colour_uniform_, red, green, blue, 1.0);
+
+	glBindVertexArray(drawing_vertex_array_);
+	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+}
diff --git a/Outputs/CRT/Internals/Rectangle.hpp b/Outputs/CRT/Internals/Rectangle.hpp
new file mode 100644
index 000000000..b98e9a4df
--- /dev/null
+++ b/Outputs/CRT/Internals/Rectangle.hpp
@@ -0,0 +1,41 @@
+//
+//  Rectangle.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 11/07/2018.
+//  Copyright © 2018 Thomas Harte. All rights reserved.
+//
+
+#ifndef Rectangle_hpp
+#define Rectangle_hpp
+
+#include "OpenGL.hpp"
+#include "Shaders/Shader.hpp"
+#include <memory>
+
+namespace OpenGL {
+
+/*!
+	Provides a wrapper for drawing a solid, single-colour rectangle.
+*/
+class Rectangle {
+	public:
+		/*!
+			Instantiates an instance of Rectange with the coordinates given.
+		*/
+		Rectangle(float x, float y, float width, float height);
+
+		/*!
+			Draws this rectangle in the colour supplied.
+		*/
+		void draw(float red, float green, float blue);
+
+	private:
+		Shader pixel_shader_;
+		GLuint drawing_vertex_array_ = 0, drawing_array_buffer_ = 0;
+		GLint colour_uniform_;
+};
+
+}
+
+#endif /* Rectangle_hpp */