Hi Rick,

Both threads are trapezoidal (not square) threads and the main difference is the angle. 29 degrees for SAE and 30 degrees for metric.
SAE thread is called ACME and metric is called Trapezoidal even recently both of them are called acme threads.
Check here: Acme screw pitch gauges metric - Duchy Fasteners
They call these gages: Acme screw pitch gauges metric and Acme screw pitch gauges imperial
On my gages is written: 29 ACME SCREW PITCH GAGE and 30 ACME SCREW PITCH GAGE.
I needed them to find out thread of my lathe lead screw and found that my lathe is
full metric with 20 mm dia x 2mm trapezoidal thread lead screw, not 3/4" x 12 TPI as I was
thinking before.
Hope this helped.
Regards
LMMM