Drag the letters of a word to set the space between each pair until the rhythm reads evenly, then compare your kerning against a typographer's.
A word sits on a canvas with a density overlay shading the whitespace between glyphs. Slide letters left or right to even out the spacing, then check your work against the ideal kerning.
Tighten trains the designer's skill of kerning by feel, spacing letters until they look right.
Adjusting the whitespace between glyphs so it reads evenly, with a density overlay, is kerning.
Showing the density rather than the letters is the original device.
Kerning is judged by area, not by distance, which is exactly what the overlay makes visible. Equal gaps between letters look wrong because shapes like A, V and T enclose space differently — so the eye wants equal apparent space, and the measurement has to be of the gap's weight.
Tighten is a typography game that trains the designer's skill of kerning by feel. It is an original, free browser game built in-house.