Vernier, dial and digital calipers can be used with accessories that extend their usefulness. The earliest angular readouts were from open vernier scales directly visible to the eye. Most sextants also include a vernier on the worm dial that reads to 0.1 minute. The guidance system was inertial with radio control of the vernier thrusters. Sometimes Vernier scales are attached to give more precise readings. The problem was traced to a malfunction of the vernier LOX poppet valve. Vernier acuity measures the ability to align two line segments. This setting is called differential delay line or vernier delay line. These same engines served as the last stage's vernier thrusters. Twenty four spaces on the Vernier scale equal in extreme length twenty three double degrees. The retrograde vernier also extends backwards along the data scale. This ability to detect alignment is called "Vernier acuity". The circles were read with a vernier scale. This section includes references to techniques which use the Vernier principle to make fine-resolution measurements. The number of this coinciding vernier line represents the additional digit.