The "Angle Mod" is an ergonomic mod which makes it possible to adopt a more comfortable typing posture by introducing a degree of symmetry on a standard, staggered keyboard. It was first developed in 2007 by Colemak users seeking to improve the experience of using that layout on such keyboards, but the principle can be applied to any layout.
To apply the mod, the left-hand keys on the bottom row are moved one space to the left, but you should continue to type these keys with the conventional finger, i.e. Z=pinky, X=ring, C=middle.
There is small difference in implementation depending on whether the keyboard is an ISO or ANSI type. The Angle Mod works best on ISO keyboards due to it having an extra key to the left of Z. On ANSI keyboards, the Angle Mod also works well, but requires an additional adjustment: most commonly, the rare Z key is relocated to the spare key in the centre of the keyboard, but other options exist - see below for examples.
Try it for yourself: put your left-hand fingers in the home position (over A S D F on a Qwerty keyboard). Then, from that position, try typing the letter C first with your index finger, then with your middle finger. You should find is it much easier and more comfortable with the index finger. Despite being almost universally taught, the traditional technique is significantly inferior, requiring either an excessive movement of the left wrist, or an awkward stretch for the finger. The Angle Mod solves this problem by allowing a more ergonomic technique to be used.
An additional benefit of the Angle Mod is it provides a huge improvement for the (Qwerty) B key, arguably the worst positioned key on the keyboard.
An alternative to using the Angle Mod is to simply use the "wrong" fingers without changing mappings, i.e. Z=ring, X=middle, C=index. This is sometimes known as the "Angle Cheat" method! For some layouts, this may be an acceptable solution, but it should be noted that most modern layouts, including Colemak, are designed assuming the conventional finger assignment. Consequently, to make the most of such layouts, adopting the Angle Mod is hugely recommended.
A version of the Angle Mod is a prerequisite for Colemak Mod-DH.
Some example layouts for different keyboard types are shown below. Home keys are highlighted with a red box. The finger that should be used is indicated by the background colour of the key.
1. ISO Keyboard:
2. ANSI Keyboard: