How to insert horizontal line to page headers in Apple Pages

Solution 1:

Insert a line, move it on the page into the header box (this is easier if you have layout visible - VIEW / SHOW LAYOUT)

Make the line a section master (ARRANGE / SECTION MASTER / MOVE OBJECT TO SECTION MASTER)

Solution 2:

I did the following:

 1) Click anywhere on paragraph section (not the header section)

 2) Go to Format on sidebar and select Layout tab

 3) Expand Borders & Rules (click upside down triangle)

 4) Select Solid Line from dropdown list

 5) Select desired Position (above paragraph)

 6) adjust the offset distance from top of paragraph if u wish
    adjust thickness if u wish