These fonts represent translation to PostScript Type 1 of the
ESSTIX fonts. ESSTIX seem to have been a precursor to the STIX
project, and were donated by Elsevier to that project. The
accompanying virtual fonts with customized metrics and LaTeX
support files allow their use as calligraphic, fraktur and
double-struck (blackboard bold) in maths mode.

Homepage:
https://ctan.org/pkg/esstix
