This collection contains implementations for aspects of the LaTeX3 kernel, dealing with higher-level ideas such as the Designer Interface. The packages here are considered broadly stable (The LaTeX3 Project does not expect the interfaces to alter radically). These packages are built on LaTeX2e conventions at the interface level, and so may not migrate in the current form to a stand-alone LaTeX3 format.

Packages provided are xparse, which provides a high-level interface for declaring document commands xfp, an expandable IEEE 754 FPU for LaTeX, l3keys2e, which makes the facilities of the kernel module l3keys available for use by LaTeX 2e packages, xtemplate, which provides a means of defining generic functions using a key-value syntax, and xfrac, which provides flexible split-level fractions.