Excerpt from an article by the late Barry Jones, a lifelong Working Terrier Man & former President of the Parson Russell Terrier Club.

Why Span

The chest measurement determines whether a PRT can follow the fox, its traditional  quarry, to ground. However large, I have never encountered a fox greater than 14 inches chest circumference in a weight range of 10lbs to 24lbs.
I cannot recall any guidelines for spanning a PRT apart from the average hand size, which of course does not exist!

How to Span

With so many variations of hand size, a more precise statement of the average hand span is required. Take a round object of four and a half-inch diameter, place your two thumbs and index fingers together forming a circle around it - this is your span. 
Some fingers will not be able to cover the entire circumference while others will have an overlap. Either way, when spanning the person should bear in mind their +or -- factor and be able to gauge the four and a half-inch diameter, which is approximately 14 inches circumference. This size of the chest in a terrier of 12inches --14inches height will look and be balanced, allowing the heart and lung room, with the flexibility to follow its quarry in underground work.
Regarding the shape of the chest -- this should not, of course, be round (barrel-chested), rather the shape of an egg in profile, large end uppermost, allowing free movement of the forelegs.

Where to span

Approach the terrier with its full knowledge of being handled -- do not grab or suddenly place hands. With the terrier on the ground, no more than the forelegs should be lifted off the floor, place hands in a direct line down around the chest directly behind the elbows.
The lifting of terriers off the ground, sometimes to chest height, and in extreme cases, causes distress and tenseness.

Taken from Original PRT Club Website First uploaded by Matt Smith

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