BEM (block, element, modifier) is a well-known method of naming components. Previously, we used the double-dash style for BEM notation (.slds-block--modifier). Double dashes are problematic in markup, which doesn't allow double dashes within comments, so we've upgraded our syntax to use the single underscore style (.slds-block_modifier). This change is backwards compatible for 18 months. All components going forward use the single-underscore BEM style.
For more information about Lightning Design System’s use of BEM, see the Lightning Design System site.
For more information on the BEM notation change and other changes, see the complete Release Notes for Lightning Design System.
The Lightning Design System is a CSS Framework that provides a look and feel that’s consistent with Lightning Experience. The component classes enable you to build custom applications with a UI that is consistent with Salesforce, without having to reverse engineer our styles.