Every dashboard I have shipped since 2019 sets its data in a monospace face, and every design review since 2019 has asked why. The honest answer is not aesthetics. It is that a fixed-width glyph turns the whole screen into a grid I get for free, and grids are what operators actually read.
Watch someone work a playout console for an hour. They do not read labels. They look at the same screen coordinates they looked at last time, because the value they care about lives there. Proportional type breaks that contract quietly: a timestamp grows a character, the row reflows, and the eye’s landing zone is now the wrong column. In monospace nothing moves unless the data means something moved. That property has a name in broadcast — a stable raster — and it is the property every glass cockpit is built around.
Numbers are the sharpest case. Tabular figures exist precisely because proportional fonts got this wrong, but tabular figures fix only digits. A monospace face extends the deal to the minus sign, the decimal point, the unit suffix — the whole token. A column of bitrates ticking ten times a second in a proportional face shimmers; the same column in monospace sits still and lets the one changed digit do the talking. The change is the signal. Everything else is noise you paid rendering time to draw.
There is also a quieter benefit: truncation becomes arithmetic. When a cell is exactly forty characters, “will this fit” is a question the data layer can answer without measuring text — no canvas, no off-screen span, no layout thrash. The terminal rewrite I wrote up earlier this year leans on this everywhere; half its performance story is that string length and pixel width are the same number scaled.
Monospace loses where text is prose. Long-form reading wants proportional type and always will; nobody should ship a settings page in a terminal face because the brand guide looked cold. The rule I use is boring: if the user reads it, set it proportional; if the user scans it, set it monospace. Interfaces are mostly scanned. That is the case, and it fits in a sentence: monospace is not a style, it is a layout engine that costs nothing.