Conversation so far:
Please explain Theatrum Orbis Terrarum?
Ortelius' Theatrum Orbis Terrarum, the "first modern atlas" (of 53 maps).
Did Abraham contribute to this?
In 1573 Ortelius published seventeen supplementary maps under the title Additamentum Theatri Orbis Terrarum.

Question: What else is important to know about this ?
Self-contained rewrite: What else is important to know about Theatrum Orbis Terrarum besides publishing maps ?