While unions come in all shapes and sizes, there are distinctions that are useful to make. There are, broadly speaking, four kinds of unions. First, craft unions. These organize with workers based on the skill involved or the kind of work they do – for example, carpenters being part of a woodworkers’ union. Craft unions may organize workers across different industries but tend to be smaller and concerned with setting boundaries around who can do their kind of work. Controlling this can be part of developing bargaining power. It also means they might work alongside workers who are not eligible to join the same union. Second, industrial unions organize with workers across one industry, regardless of the level or job of the worker. The strength of this model comes from the ability to strike to shut down the whole supply chain. The IWW is one example of radical industrial unionism. While they accept all members, they focus on organizing whole industries, like factories, mining, utilities and transportation. Third, professional unions are like craft unions, but they have grown in so-called white-collar professions. For example, academic workers in the UCU (University and College Union) in the UK is only open to academic or academic-related workers in universities and colleges, not to the wider workforce. Fourth are the general unions that recruit workers from any industry or sector. Most unions today are closer to this kind of model, and many of them are the result of mergers. For example, the SEIU (Service Employees International Union) in the US or Unite in the UK. These unions can grow to be very large, with many having membership in the millions. They may have a blend of craft, professional and industrial unionism within them, depending on their history.