The first step that people usually take in applying for a patent is to file a preliminary application in one country. When the application is filed, the date of application is recorded and this is called the “priority date”. The first application can be quite basic and does not have to include a set of claims (see below). It is still an important document and specialist advice from a patent agent should be obtained in preparing it.
Most countries are signatories to an international convention, which guarantees that the priority date of an invention filed in one country will be respected in other countries, provided an application is filed in the other countries within twelve months of the date of filing the first application.
This is why the first document filed can be very important later. The system of filing an application in one country initially can be of great benefit to inventors provided they have timed it correctly. It allows up to twelve months before foreign applications must be filed.
During this time the inventor can assess the commercial prospects of the invention, carry out improvements on it, and arrange the necessary finance for international patenting and commercial exploitation through manufacture and sale. This period is also used to assess the market potential for the invention in various countries and to decide in which countries the expense of patenting is justified.
Note though the comments earlier about the dangers of underestimating the time it takes to do these things and the dangers of filing too early.