Tube latches, mortice latches and the bolts that make your handles actually function.
The handle is what you see. The latch is what works. Get the latch wrong and the door will not close properly no matter how good the hardware on the face of it is.
Backset
The single number that matters. Backset is the distance from the edge of the door to the centre of the handle spindle hole. 60mm is the most common here, but 57mm and 45mm both appear.
Measure the existing latch before ordering a replacement. If the backset does not match, the new handle will not line up with the hole already in the door.
Latch or lock
A tube latch holds the door closed, no key. A privacy set adds a snib. A mortice lock takes a key or euro cylinder and is what you want on an external door.
Buying as a kit
Iver handles can be bought as passage, privacy or entrance kits with the correct latch already included. Unless you have an existing lock to work around, the kit is the simpler path.
Call 1300 011 268 or email hello@ellagrace.com.au.