You might already know about Underscores, often abbreviated as _s, which is a popular starter theme for WordPress. This is a big project maintained by
New Course: WordPress Theme Creation With Underscores
How to create a theme with underscores (_s) - Speaker Deck
Underscores.me — The Best Way To Get Started With The _s Theme
How to create a theme with underscores (_s) - Speaker Deck
60 Best Responsive WordPress Themes (Free and Paid) - Hongkiat
Create a blank WordPress theme using Underscores
Theme Development From Scratch w/ Underscores & Foundation – DobsonDev
Browse Open Source WordPress Frameworks on IncludeWP
WordPress Theme Development: Getting Started with Underscores (_s