When it is safer for the tenant to vouch a warrantor than to undertake the defence in  his own person, since no exception, peremptory or dilatory, wherewith to destroy the  action completely or defer it temporarily, lies for him, then, if he has a warrantor, let  him vouch him at once, sometimes by the aid of the court sometimes without it, according  as the warrantor is within the vouchor's potestas, so that he may have him  without aid, or within another's, so that he cannot produce him without aid.1  Within another's potestas, I say, as within the potestas of the lord king, in the realm of  England or in Ireland or in Wales, in some place under the dominion of the lord king.  If he who is vouched is outside the potestas of the lord king, he will be vouched in  vain by aid of the court, because the tenant will then have to produce him himself, if  he can, otherwise he will lose.2
What warranty is.
 We must first see what warranty is. It is clear that to warrant is nothing other than to  defend and acquit the tenant who has vouched the warrantor in his seisin, for though  the warrantor warrants, seisin is not thereby transferred to him, nor is anything  transferred except the defence of the thing claimed, which defence, when it has once  been given over to the warrantor, cannot thereafter be resumed by the tenant, everything  proceeding in the name of the warrantor; nor may he, after he has warranted,  render the thing warranted to the demandant against the tenant's will or without  judgment, without committing a disseisin, since to defend does not mean to render  without the tenant's consent. But when the tenant consents or a judgment is given,  seisin of the thing claimed is at once transferred to the demandant and the tenant may  have escambium to the value from the warrantor's property.
Who may vouch a warrantor and on what grounds.
 We must also see who may vouch a warrantor and on what grounds. It is clear that  everyone who is not forbidden may vouch a warrantor,