[Proj] where is the default origin ?

[Randy]

Hi list,
I'm new here, and know little about projection in GIS.

I'd like to know where the default origin is when I do Mercator
projection like this:
from "+proj=latlong+ellps=WGS84+datum=WGS84"
to   "+proj=merc+lat_ts=30+ellps=WGS84+datum=WGS84+unit=meter"

I just know the default origin is the point of intersection with
greenwich and equatorial when I use the default latitude of true
scale,but if now the latitude of true scale is decided by "lat_ts",where
is the default origin?
 
Thanks a lot!

Best regards,
Randy