The age and make of your classic car determines what sort of spanners, sockets, nuts and bolts you need for it. A British-manufactured classic from the 1960s up to the 1980s will probably use unified fastenings, with the most popular spanner/ socket sizes being 1/2in, along with 9/16in and 7/16in. By the 1990s, many of these cars were using metric fastenings (measured in millimetres, or mm for short), which the continental manufacturers had already been using for a few decades. Nowadays the Americans have largely retained unified sizes, but most other manufacturers have adopted metric.
Before unified was introduced, there were the likes of Whitworth and BA (British Association), which you'll find on pre-war British classics. Moving to a newer standard of thread was made easier in some cases by…