They seldom count in words above 4, and almost never as high as 7.

One of the most careful observers among them expresses his doubt as to a native's ability to discover the loss of two pins, if he were first shown seven pins in a row, and then two were removed without his knowledge.[168] But he believes that if a single pin were removed from the seven, the Blackfellow would become conscious of its loss.
This is due to his habit of counting by pairs, which enables him to discover whether any number within reasonable limit is odd or even.

Some of the negro tribes of Africa, and of the Indian tribes of America, have the same habit.

Progression by pairs may seem to some tribes as natural as progression by single units.

It certainly is not at all rare; and in Australia its influence on spoken number systems is most apparent.
Any number system which passes the limit 10 is reasonably sure to have either a quinary, a decimal, or a vigesimal structure.