On May 19, 2003, Paul Anastas along with his future wife, Julie Zimmerman created the 12 Principles of Green Engineering. This expanded upon the 12 Principles of Green Chemistry to not only include the guidelines for what an environmentally conscious chemical should be in theory, but also what steps should be followed to create an environmentally conscious alternative to the chemical. Environmentally conscious thought can be applied to engineering disciplines such as civil and mechanical engineers when considering practices with negative environmental impacts, such as concrete hydration. These principles still were centered around chemical processes, with about half pertaining to engineers. There are many ways that both the 12 Principles of Green Chemistry and 12 Principles of Green Engineering interact, referred to by Tse-Lun Chen et al. as "cross connections". Every one Principle of Green Engineering has one or more corresponding "cross connections" to Principles of Green Chemistry. For example, principle 1 of green engineering is "Inherent Rather than Circumstantial", which has cross connections to principles 1, 3, and 8 of green chemistry.