Instead of applying `.btn-` or `.input-` to individual elements in
button and input groups, we now have new sizing classes to keep your
markup a little leaner.
* Add `.input-group-sm` or `.input-group-lg` to your `.input-group` to
replicate `.input-sm` and `.input-lg`, respectively.
* Add `.btn-group-xs`, `.btn-group-sm`, or `.btn-group-lg` to your
`.btn-group` to replicate `.btn-xs`, `.btn-sm`, and `. btn-lg`,
respectively.
Both sets of new sizing classes simply use the existing individual
sizing classes as mixins, so customization of them is automatically
inherited when compiling.
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