1 INTRODUCTION

2 DATA

2.1 An arm-like coordinate system

3 OBSERVED PROPERTIES OF THE NORTHERN ARM

3.1 Metallicity

3.2 Structure

3.3 Kinematics

4 MODELLING AND ANALYSIS

4.1 General methodology

4.2 N-body model

4.3 Simpler model suites

4.4 Simple model kinematics along the northern arm

4.5 Origin of the northern arm

5 SUMMARY