On April 4, 2018, at 1229 mountain daylight time, a Piper PA-12, N7709H, collided with a Diamond Aircraft Inc. DA42 NG, N621ZC, during taxi at Bozeman Yellowstone International Airport (BZN), Bozeman, Montana. Both airplanes sustained substantial damage. Neither the student pilot or the flight instructor in the PA-12, nor the flight instructor and pilot receiving instruction in the DA-42 were injured. Both airplanes were operated as Title 14 Code of Federal Regulations Part 91 instructional flights. The pilots of the DA-42 were conducting an instrument approach for runway 12 and the tower controller cleared the airplane to land. The PA-12, which had been conducting touch-and-go takeoffs and landings in the pattern for runway 11, requested to switch to runway 12 and was instructed by the tower to follow the DA-42 for landing on runway 12. Shortly thereafter, the controller contacted the PA-12, advised that they were unable to approve a landing on runway 12, and cleared the airplane to land on runway 11. The PA-12 subsequently landed on runway 11 and was instructed to taxi back to the approach end. Over the next minute, the tower controller provided a landing clearance to another airplane that was on the right downwind leg for runway 12, and then instructed the DA-42, which had landed, to exit runway 12 and contact the ground controller. The DA-42 exited the runway at taxiway A3 and stopped short of taxiway A. At this time, while two airplanes were now in the final approach stages for landing on runway 12, the tower controller cleared the PA-12 to cross runway 11 and hold short of runway 12. Shortly thereafter, the controller cleared the PA-12 to cross runway 12 and contact the ground controller. At this time, the DA-42 was still stationary on taxiway A3, and about 30 seconds later, the controller exclaimed, "09H, STOP;" however, the PA-12 collided with the tail of the DA-42. At the time of the collision, the DA-42 was positioned about 220 ft beyond the runway 12 hold short lines (440 ft from the edge of runway 12). According to the instructor onboard the DA-42, after landing on runway 12, they were instructed to exit at taxiway A3 and hold short of taxiway A. After exiting at A3, he stated that they stopped short of A, and just as they were switching to the ground control frequency, they felt a collision. Neither pilot onboard the DA-42 saw the airplane that struck them. Both the student and instructor onboard the PA-12 stated that the tower controller issued a clearance to cross runway 12 while they were still holding short of runway 11. As such, they felt pressure to perform a fast taxi and did not execute the typical "S"-turns for enhanced forward visibility. Air traffic control audio recordings revealed that the tower controller issued the PA-12 with progressive instructions to hold short of and cross runway 11, and subsequently, to hold short of and then cross runway 12. Those instructions were read back correctly by the student as the taxi progressed, and at no time did the tower controller provide the PA-12 with instructions to expedite the taxi. Both occupants stated that they did not see the DA-42 on the taxiway. The instructor did notice the student pilot glance down at the instrument panel to switch to the ground control frequency just before the impact. The airport was equipped with four runways, including the parallel runways 12/30 and 11/29. Taxiway A3 branched from runway 12, 4,300 ft beyond the displaced threshold. It was 630 ft long and connected runway 12 to the parallel taxiway A. Taxiway C3 was directly in line with A3 and connected the landing threshold of runway 11 to runway 12 at midfield. (see Figure 1.) The Piper PA-12 was a high-wing, single-engine airplane with a tandem seating configuration and was equipped with conventional (tailwheel) landing gear and tundra tires. The student pilot was seated in the front seat, and the instructor in one of the rear seats. The instructor stated that the forward visibility from the front seat was limited and that the forward visibility from the rear seats was even worse. Figure 1. Landing and Taxi Paths