Invoking the centuries-old - and extremely rare - law of ''praying a tales'', sheriff's officers were dispatched last week into Civic to track down a spare juror.

The issue came to a head during a jury selection process last week after potential jurors were whittled down to just 11. The jury eventually formed was discharged yesterday - for legal reasons completely unrelated to the pray a tales rule - when Justice John Burns found it was impossible to proceed with the trial.

The accused, Adam John Street, has denied a charge of aiding and abetting an aggravated robbery and the hearing is expected to begin again with a new jury later this year.

The discharge came seven days into the trial, and the judge thanked the jurors for their service to the community.

''I'd particularly like to thank the member of the jury who was shanghaied off the street to participate in this trial,'' Justice Burns said.

He added it must have been ''much more inconvenient'' for the individual to be pulled away from his daily routine. Court staff stopped the person during work hours near the legal precinct last week, and after necessary formalities the 12th juror was directed to show up for jury duty the next morning.

''Praying a tales'', a legal Latin-French term, roughly translates as ''praying that the number of jurymen be completed''. It's an ancient term harking back to a time when a jury of peers would be culled from villagers and, should some be deemed unsuitable, others would be brought in off the street. Jurors picked in such a fashion were known as ''talesmen''.

But in contemporary criminal justice systems it has become a rarely used - although not unheard of - legal phenomenon.

Canberra barristers with long memories have suggested the issue may have arisen in the time of former Chief Justice Jeffrey Miles, who sat on the bench from the mid-80s to 2002, although The Canberra Times was unable to confirm this.

Modern jury panels are selected from the electoral role and whittled down, with lawyers on both sides of the bar table able to ''challenge'' potential jurors.

And territory law allows certain people to claim exemptions from jury service: for example nurses, teachers or people 60 years or older.

But, while the court must give at least a week's notice to people called up for jury duty - or four days if they're told in person - the power to pray a tales still remains.

A ''shanghaied'' jury member is believed to be entitled to the same right to claim exemption as another citizen.