The evolution of window and class extra bytes in Windows

- Raymond Chen is a veteran Microsoft developer who blogs at "The Old New Thing" about Windows history and internals. His posts often assume deep knowledge of Win32 programming. - "Window extra bytes" and "class extra bytes" refer to small chunks of memory that old-school Windows programmers could reserve when registering a window class (a template for creating windows). These bytes let you store per-window or per-class custom data without creating your own data structures — a trick from the 16-bit era when memory was tight and C++ wasn't yet the norm. - The "prefix" mentioned is the naming convention used for these byte offsets (e.g., `cbWindowExtra` vs. `cbClassExtra`), which signals whether the extra bytes belong to each individual window instance (window extra) or are shared across all windows of that class (class extra). - This matters because it's a piece of Windows archaeology: understanding how developers worked around the lack of proper per-window data storage before `SetWindowLongPtr` / `GWLP_USERDATA` and modern C++ object-oriented approaches became standard.