CUSTOMS UNION
A FTA evolves into a customs union, which is regarded as a stronger form of co‐operation. Customs union provides for economic cooperation as in a free-trade zone and barriers to trade are removed between member countries, and members agree to treat trade with non-member countries in a similar manner. The primary difference from the free trade area is that members agree to treat trade with non-member countries in a similar manner. The Gulf Cooperation Council (GCC) is an example.
According to Markussen et al (1995), who describe a customs union as co‐operation that eliminates all trade barriers among member states of the union, but imposes a common tariff against non-member countries. Generally speaking, a customs union combines the elements of a FTA with policies of protection, since it provides for freedom of movement of goods between member states, while on the other hand it protects the market within the union from competition from other countries.

COMMON MARKETS
When the economic integration process extends beyond the elimination of trade barriers to the movement of factors, this stage is called the common market. A common market is defined as a group of trading nations that permits: (i) the free movement of goods and services among member nations; (ii) the initiation of common external trade restrictions against non members; and (iii) the free movement of factors of production across national borders within the economic bloc (Carbaugh 2004). It