A 9" Ford rear was introduced in 1957 and used for over 2 decades in trucks and many cars.
A QC, or Quick-change, rearend is based on the early Ford (pre-49) rearend and featuires a center housing which has a rear cover which allows gear changes to change the ratio of the rearend. There are now many QC rearends available some of which are modified to use later axles and other parts. Halibrand is the most notable and familiar name. Others are Winters, Frankland, Cyclone.
A quick change rear has a cover on the rear side, under it is 2 spur gears, one small and one larger, one above the other. By swapping their positions, the gear ratio is changed. The qc also has a conventional type ring and pinion. The driveshaft input goes past the ring gear to the spur gear, then the second spur gear turns the conventional pinion and ring gear.
Let's keep this simple.
If the ring and pinion has a 4.00/1 ratio...... and the spur gears are say 30 and 28 teeth ..... If the driveshaft gear is 30 then the spur ratio is 28/30= .93 the overall gear is .93 x 4.00= 3.72
If we pop the cover and swap the qc gears top to bottom then the driveshaft gear is the 28 teeth, then the ratio is 30/28 = 1.07 then the overall ratio is 4.00 x 1.07 = 4.28
We can "quick change" the spur gears and have either ratio in minutes. No ring and pinion work required.
Multiple ratios are available, this was just a simple example.
Just think. If you had a 30/27 gearset, you could have a 4.44 for the drags, and 3.6 to drive home with.