Overshoot refers to the slight extension of curved or pointed glyph parts beyond a given height or alignment guideline (such as the baseline, x-height, or cap height).

For example:
• the round part of O often extends slightly above the cap height and below the baseline.
• the pointed part of A might go a tiny bit above the cap height.

This is done optically, so that curves and points can appear visually aligned with the flat shapes (such as H or E) even if—mathematically speaking—they go beyond past the guidelines. Without overshoot, rounded or pointed shapes would appear smaller or misaligned to the eye.

FONT ENGINEERING HINT

Overshoot values are used in hinting and can also be reflected in font metrics (such as vertical alignment zones) to ensure consistent rendering across sizes and platforms. On a small pixel grid, overshoot can make curved and flat segments appear uneven. Hinting instructs rasterizers up to which size the overshoot should be suppressed, so curves and flat parts appear visually even at any sizes, and allows it to be restored at larger sizes where the difference is no longer visually noticeable.

In the .otf font format files, the overshoots are stored as BlueValues and OtherBlues in the CFF (Compact Font Format) table. This is because these guidelines are often represented as areas colored in blue in font editors. In .ttf files, the overshoots values are stored in the cvt (Control Value Table) table.