Fluid grid with fixed gutters, based on fractions rather than column widths. Thinking in fractions allows for infinite columns and nesting. No matter the context, fractions are always true.

Use a common denominator to stay on a specific grid (e.g. 1/2, 1/6, 1/12) - or wander off-grid in a heartbeat with non-divisable fractions (e.g. 1/5, 1/7).

Inspired by http:https://builtbyboon.com/blog/proportional-grids