Medium (MD)

Viewport smaller than 1140px. CSS media query:

@media (max-width: 1139px) { ... }

Class prefix (where available): md-. Example: md-pt20

Small (SM)

Viewport smaller than 768px. CSS media query:

@media (max-width: 767px) { ... }

Class prefix (where available): sm-. Example: sm-pt20

This is the default breakpoint in HubSpot (Bootstrap 2), when the columns stack one after another on mobile.

Extra Small (XS)

Viewport smaller than 480px. CSS media query:

@media (max-width: 479px) { ... }

Class prefix (where available): xs-. Example: xs-pt20