Being involuntarily out of work. Structural unemployment occurs when the labour market no longer requires a particular skill; in the case of fractional unemployment, jobs are available but not taken up because of immobility or the lack of information. Suedekum (2005) Papers Reg. Sci. 84, 2 shows that a core–periphery in real wages is associated with and magnified by regional unemployment disparities. For Italy, M. F. Cracolici et al. (2007) explain spatial variations in unemployment differentials in terms of spatial equilibrium and disequilibrium factors, and a significant degree of spatial dependence among labour markets at the provincial level in Italy. Additionally, provinces marked by high, or low, unemployment tend to be spatially clustered. Writing on Spain, López-Bazo et al. (2002) Papers Reg. Sci. 81, 3 also find evidence of clustering. Hämäläinen (2002) Papers Reg. Sci. 81, 4 suggests that regional measures to reduce unemployment may cut out-migration, but only during an era of low unemployment. Christopoulos (2004) Papers Reg. Sci. 83, 3 confirms Okun’s law: that unemployment falls with increases in the ratio of real output (GDP) growth to potential real output growth. The unemployment rate is the number unemployed as a percentage of the total population of working age.