A tie line, also known as a tie trunk, is a communication connection between extensions of a private telephone system, typically two PBXes. ... Found on http://en.wikipedia.org/wiki/Tie_line
A horizontal line constructed across a two phase region of a binary phase diagram; its intersections with the phase boundaries on either end represent the equilibrium conditions of the respective phases at the temperature in question. Found on https://metallurgyfordummies.com/metallurgy-glossary.html