A delivery system , comprising: a catheter having a distal end and a proximal end ; a balloon portion positioned at the distal end of the catheter ; a medical device , having a compressed state and an expanded state, positioned about the balloon portion ; and a sheath positioned about the medical device to hold the medical device in the compressed state, the sheath comprises: a distal sheath portion located at a distal end of the sheath , the distal sheath portion having a first diameter and a first longitudinal length; a transition portion , of a second longitudinal length, having a distal end located adjacent the proximal end of the distal sheath portion, the distal end of the transition portion being of the first diameter and having a proximal end with a second diameter greater than the first diameter; a body portion of a third longitudinal length, having a distal end adjacent a proximal end of the transition portion , the body portion being of the second diameter; an opening provided in an outer surface of the sheath , a distal-most end of the opening being located at a predetermined distance proximally from the distal end of the sheath ; characterized in that the sheath comprises material with a grain oriented along a longitudinal axis of the sheath, and wherein the opening is an initiation slit of a predetermined length oriented substantially in parallel with the material grain. a catheter having a distal end and a proximal end ; a balloon portion positioned at the distal end of the catheter ; a medical device , having a compressed state and an expanded state, positioned about the balloon portion ; and a sheath positioned about the medical device to hold the medical device in the compressed state, the sheath comprises: a distal sheath portion located at a distal end of the sheath , the distal sheath portion having a first diameter and a first longitudinal length; a transition portion , of a second longitudinal length, having a distal end located adjacent the proximal end of the distal sheath portion, the distal end of the transition portion being of the first diameter and having a proximal end with a second diameter greater than the first diameter; a body portion of a third longitudinal length, having a distal end adjacent a proximal end of the transition portion , the body portion being of the second diameter; an opening provided in an outer surface of the sheath , a distal-most end of the opening being located at a predetermined distance proximally from the distal end of the sheath ; characterized in that the sheath comprises material with a grain oriented along a longitudinal axis of the sheath, and wherein the opening is an initiation slit of a predetermined length oriented substantially in parallel with the material grain. a distal sheath portion located at a distal end of the sheath , the distal sheath portion having a first diameter and a first longitudinal length; a transition portion , of a second longitudinal length, having a distal end located adjacent the proximal end of the distal sheath portion, the distal end of the transition portion being of the first diameter and having a proximal end with a second diameter greater than the first diameter; a body portion of a third longitudinal length, having a distal end adjacent a proximal end of the transition portion , the body portion being of the second diameter; an opening provided in an outer surface of the sheath , a distal-most end of the opening being located at a predetermined distance proximally from the distal end of the sheath ; characterized in that the sheath comprises material with a grain oriented along a longitudinal axis of the sheath, and wherein the opening is an initiation slit of a predetermined length oriented substantially in parallel with the material grain. <EOS>