No, it's like Jacek said. It'll be easier to explain it with ex - if you have a font that is 12pt high then 1ex will be about 8pt. As "m" is the widest character for proportional fonts, the width of "m" will have a larger value (in points) than the width of "i", so you can't just say that your font is "8 points wide" (unless the font is monospaced, like Courier), but you can say that its "m" glyph is 12 points wide.