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 type these keys with the conventional finger, i.e. Z=pinky, X=ring, C=middle. It works best on an ISO keyboard due to it having an extra key to the left of Z, but is nonetheless possible on ANSI keyboards too with an adjustment.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 the fingers to twist in an awkward way.
One possible solution to this is to simply use the "wrong" fingers, i.e. Z=ring, X=middle, C=index. 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.
An additional benefit of the Angle Mod is it makes the Qwerty B key, arguably the worst positioned key on the keyboard, much easier to reach.
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: