====== Global Multi-Domain Design ======
When a company spans more than one continent, I design the active directory forest based on a geographical model:

<code>
Root (Used for adminstration accounts and "universal" groups; but otherwise empty)
 |
 +--- americas (includes North America and South America)
 |
 +--- emea (includes Europe, Middle East, and Africa)
 |
 +--- apac (includes Asia, Australia, and any countries in the Pacific Ocean)
</code>

If the number of objects (users, groups, OUs) in one domain exceed 250,000, I typically would break the geographic domain down to country domains such as:
<code>
americas (This becomes an "empty" domain and is used to control replication to/from the "forest")
 | 
 +---us
 |
 +---canada
 |
 +---brazil
</code>

Each of these domains would then follow my design for single-domain designs.