When A. Lange & Söhne was preparing to relaunch, they decided to make a splash with a truly unique watch, the Lange 1. The hallmarks of that watch, and this movement, are the asymmetric dial arrangement and two-window big date. This latter complication was designed to resemble the 1841 Five-Minute Clock at the Semper Opera House in Dresden and gave the new manufacture its first patent. Although the dial is not centered, it does have a pattern: The hour and minutes dial, date window divider, and subsidiary seconds dial form the point and two leg apexes of an isosceles triangle, with the power reserve indicator centered at this base.

All L901 calibres are based on this original design, and all feature the same basic complications: Big date at 1 00, power reserve indicator at 3 00, small seconds at 5 00, and hour and minute hands at 9 00. The movement is constructed with a three-quarter plate, made of untreated German silver and decorated with Glashütte Stripes, in traditional Glashütte style. It features nine gold chatons and blued screws. In all, there are 53 jewels synthetic rubies.

For an accurate rate result the screwed balance with Nivarox 1 hairspring is responsible. It is shock-proofed, makes 21,600 vibrations per hour and is adjusted in five positions. On the hand-engraved balance cock, the Swan's neck fine regulation is mounted. Its patented beat adjustment mechanism makes it possible to accurately set the impulse pin relative to the pallet fork without removing the balance wheel.

• Introduced in 1998, Cal. 901.1 was a special version of Cal. 901.0 with gold lever and escape wheel and a gold cocks for the balance, lever, and escape wheel. It was used in the limited-edition gold-dial Lange 1A, produced from 1998 until 2001. Another minor variation is Cal. 901.4, introduced in 2001, with longer pinions for watches with thicker dials, such as mother of pearl.
• Cal. 901.5 adds a moon phase display to the small seconds subdial. It was introduced in 2002 for the Lange 1 Moonphase and remains in production.
• The initial 2003 Grand Lange 1 featured the Cal. L901.2 movement, but the 2012 model features a new thinner movement, Cal. L095.1.
• Calibres L901.7 and L901.8 were introduced in 2003 for the limited-edition Lange 1 "Luna Mundi" and were produced only through 2004. These two movements are identical apart from their calibration Cal. 901.7 is calibrated for the Northern hemisphere, while Cal. 901.8 is calibrated for the Southern. They were used in the Lange 1 Moonphase “Ursa Major” and “Southern Cross”, respectively, and sold as a set in a limited edition of 101.
• A smaller variant with moon phase, Cal. L901.9, was used in the "Little" Lange 1 Moonphase in 2009 and 2010.

The tourbillon Cal. L961 is based on Cal. L901.

When the Lange 1 was redesigned in 2015, it was fitted with a new movement, Cal. L121.1.

• Lange L095 - The replacement movement for the Grand Lange 1
• Lange L121 - The replacement movement for the Lange 1
• Lange L031 - Second timezone movement for the Lange 1 Zeitzone
• Lange L961 - A derived tourbillon movement