This page discusses improved usage of modifier keys - Shift, Alt, Ctrl, Extend, Win/Super/Command, and CapsLock.
Amongst the many flaws in the standard keyboard design is the large spacebar in centre of the keyboard, with most modifiers offset to the extreme left and right. Frequently needed modifier keys are good candidates as "thumb keys" - if the key is readily accessible with a thumb, it is comfortable and easy to hold a key with the thumb while using fingers to press a secondary key.
The best-placed modifier keys on most keyboards are the keys either side of the spacebar - the Alt keys on a Windows/Linux PC or the Command keys on a Mac. The next best is probably the CapsLock key, being on the home row albeit via a weak finger (pinky). Many users remap CapsLock to something more useful such as Extend, Backspace, or Control.
For most people, the most commonly needed modifier is the Shift key. The standard way of typing shifted characters - using one hand to move diagonally down to hold shift while a finger on the other hand reaches for another key - is far from satisfactory for such a frequent action. Shift would be much better served on a very accessible key, preferably operated using a thumb. For users of the Extend Layer, this key should also be in a high priority position.
With these goals in mind, presented here are suggested alternative arrangements to common modifier keys:
Note: These configurations work especially well in conjunction with the Wide Mod.