Possible Clues:

• USC's conference
• Arizona and Arizona State joined it in '78
• Old name of a gp. that now has two members
• Bygone West Coast conference name
• College sports conference, formerly
• West Coast conference until 2011 when its number grew to 12
• Previous name for an athletic conference now with 12 members
• Conference name changed in 2011 when a pair of schools were added
• Athletic grp. that had to change the number at the end of its name with the addition of UC-Boulder and University of Utah
• Conference before the additions of Colorado and Utah