A region can be connected with external parties in several ways. There are at least five modalities of cooperation “across-regions”: inter-, extra-, cross-, trans-, and pan-regionalisms.
Existing studies on horizontal cooperation of a region often emphasize inter-regionalism, namely, region-to-region mechanism. The primary reason for this is that the majority of studies on cooperation across regions deal with the European Union’s (EU) relations with other regions (Gilson 2002; Doctor 2007; Börzel and Risse 2009; Renard 2016). However, it should be noted that the EU is unique in this context, because it has a common external policy centralized in Brussels, which diminishes the role played by individual European countries. Inter-regionalism naturally plays a dominant role in the EU’s external policy formulation. This, in turn, means that the EU is not an ideal case to examine the significance of inter-regionalism, in comparison with other forms of cooperation across regions, such as trans-regionalism.

Region and regionalism

Gamble and Payne (1996) define regionalism as a state(s)-led project designed to reorganize a particular regional space along defined economic and political lines. A region is a space that has geographical limits (Hurrell 1995). The establishment of regional organizations is one of the typical cases of regionalism because their membership gives us a clear idea about the boundaries of the region (Hettne 2003).
Regions are relative concepts; it is difficult to argue objectively which region is the most appropriate one. Hence, the fundamental question regarding the “level” of region exists. A larger/higher region (meta-region) and a smaller/lower region (sub-region) accompany any region. Moreover, there is a possibility of a sub-region being contested as a region (Hook and Kearns 1999). A large number of studies have analyzed complex, multi-layered regional systems. In the Asian context, for example, if one regards East Asia as a primary region, Asia-Pacific becomes a meta-region and Northeast Asia becomes a sub-region. In fact, Higgott and Stubbs (1995) argue that the Asia Pacific concept represented by Asia-Pacific Economic Cooperation (APEC) and the East Asia concept by East Asia Economic Caucus (EAEC) are competing. Further, Stubbs (2002) argues that the emergence of East Asia (ASEAN+3) would lead to the decline of Southeast Asia (ASEAN) and Asia-Pacific (APEC) based on competition hypothesis. Meanwhile Kuriyama (2012) argues that Trans-Pacific Partnership (TPP) and APEC, can mutually reinforce each other, despite the latter including countries such as China and Indonesia that do not have TPP membership, based on complementary hypothesis.
Another important angle in analyzing regionalism, which has not attracted as much attention as the multi-layered regional system, is the perspective of “across regions.” The relation of any given region with other region(s) is the question. As Renard (2015) argues, a perspective beyond regions should be taken to fully understand the complex web of international and regional
systems. Using the case of the EU, his analysis examines the compatibility among regional (EU), extra-regional (EU-external partners), inter-regional (EU-Asia), and multilateralism. However, in this context, the exceptional character of the EU with its centralized external policy should be taken into account. An extension of the work of Renard is necessary to have a systemic analysis across regions. In the Asian context, Solis and Katada (2007) made an important contribution by pointing out the fact that the proliferation of free-trade agreements (FTA) “across region,” as opposed to intra-regional FTAs, are an important phenomenon in understanding Asian integration. In fact, there are many FTAs signed between Asian and Latin American countries (such as Japan-Mexico).
It is critically important to clarify the exact definition of each modality of cooperation across regions, because there are some confusions regarding their definitions. Different scholars have defined, for example, the term trans-regionalism differently. Inter-regional and trans-regional are sometimes distinguished, while sometimes they are used interchangeably. The various forms of cooperation cannot be compared in a systematic way without clear and distinct definitions. Extending a preliminary attempt to classify the various forms of cooperation made by Hänggi (2006) and Renard (2015), the paper defines the five types of across-regional cooperation (Figure 1), namely, inter-, extra-, cross-, trans-, and pan-regionalism with examples of European cases, as follows.

Inter-regionalism is defined as region-to-region cooperation. Because a region often establishes a regional institution or group, inter-regionalism is characterized as a group-to-group relation. A typical example of inter-regionalism is that between Europe and Asia. Gilson’s (2002) Asia Meets Europe is one of the early literature that theorizes inter-regionalism.
Extra-regionalism is cooperation between a regional group and one country outside the group. An example of extra-regionalism is EU-Japan relations (Renard 2015).

Cross-regional means bilateral relations between countries that belong to different regional groups (Solis and Katada 2007). The relationship between Germany and Japan is an example in the European context, though this type of institution is less visible nowadays in Europe, which tends to employ common external policy led by Brussels.
— In the case of trans-regionalism, a new group is formed across two or more existing groups (Hangi 20006 ; Dent 2003b7 ). At least one existing group should be divided (otherwise, cooperation becomes inter-regionalism or pan-regional), though typically, two or more existing groups are divided. At least two members from the group(s) divided should join the new group (otherwise, cooperation becomes extra-regional, which is group plus one
external country). The so-called 5+5 Summit comprising of Algeria, Libya, Mauritania, Morocco, Tunisia, France, Italy, Malta, Portugal, and Spain is a typical example of trans- regionalism wherein two groups (Africa and Europe) are divided.The Ibero-American Summit established in 1991 that includes Spain, Portugal, and Latin American states is an example of trans-regionalism wherein only one group (in this case, Europe) is divided
(Grugel 2002, 8).
Pan-regional cooperation is inclusive in nature. It can cover members from many existing groups (some of them may be divided). For example, Pan-Eurasian cooperation may include Europe, Asia (East Asia, South Asia, West Asia, and Central Asia), and Russia (Vinokurov and Libman 2012, 12). It is important to note that the five modalities of cooperation across regions is an analytical framework of the web of regionalism, rather than a theory. With the five modalities, we can examine how cooperation across regions evolves across time; the rise and fall of one cooperation framework could be attributed to the rise or fall of the other frameworks.