InDesign's ruler "zero point"

Barb wrote and asked:

"I’m guessing I did something inadvertently. I’m setting up a new large project. The (single) page size is 8.5” wide. I’m setting it up as facing pages and I want the ruler to be set at 8.5” (in the middle) when at the start of the second page – instead it shows up as 0” and goes to 8.5” again. In other words I want the ruler to read from 0” to 17” for the facing pages spread. What do I need to do to change this?"

Short answer: Just right-click (or ctrl-click) on the horizontal ruler, and choose "Ruler Per Spread".

Long answer:

"Ruler Per Page" will position the zero point at the upper-left corner of each individual page.

"Ruler Per Spread" will position the zero point at the upper-left corner of the entire spread.

"Ruler On Spine" will position the zero point at the upper-left corner of right-hand (recto) pages and the upper-right corner of left-hand (verso) pages.