Many mountain ranges (Alps, Carpathians, Himalayas) and volcanic island ridges (Aleutians, Banda, Mariana) are curved or sinuous, often so regularly as to be described as arcs. Their curvature may be primary or secondary. Among numerous models proposed to account for the curvature are the orocline or secondary bending hypothesis of Carey (1958), the inextensional bending model of Frank (1968) and the spreading of marginal seas and lateral compression (Matsuda & Uyeda 1971). Different models imply different patterns of finite strain. Palaeomagnetic measurements can also be used to discriminate between them. Criteria for discriminating are examined, with particular reference to the Variscan arc in the Iberian Peninsula.