OpenType features are tedious to define using a GUI.[19] Consequently, Adobe standardized[20] a text specification format for feature files, which typically have a name ending in a .fea extension. These files can be compiled into the binary font container (.ttf or .otf) using Adobe FDK (AFDKO), FontLab or FontForge. The latter program implements a few features that are documented in the Adobe standard but are not implemented by AFDKO.[21]