This tutorial discusses the difference between the applied pressure an
d the partial pressures over the phases in the system. It develops thi
s discussion to indicate the diverse ways in which data concerning pha
se equilibria may be displayed. This is the first of what is planned t
o be an aperiodic series of tutorials. The series is not planned to be
an organized course but rather to be individual expositions of topics
relevant to the understanding and use of data concerning phase equili
bria.