Some might know about this already, others may wonder what that thing they’ve heard of before is exactly and most of you will be taken aback by the existence of any space other than the one produced by the space bar.
The space that separates words—should the text not be justified—is a fourth of the points of the letter in use. That is, when working with a 12 point letter (AKA size 12), the space between words will be 3 points; whereas using a 20 point letter (because you don’t know how to fill the amount of pages required), it will be 5 points. That’s called a four-per-em space or mid space.
The thin space, the unknown, is a sixth of the point of the letter (2 for a 12 point letter). Not that this point issue matters, for the text editor controls it. However, it is important to know when to use each space and how to insert them.
The mid space is no secret: it’s used to separate words and it’s inserted with the space bar. Make sure, nevertheless, not to use double space—although some would use it between sentences. To avoid this, use the find and replace tool. Press Ctrl + F for the find tool to appear. Then, in the replace tab, type two spaces in the find box and only one in the replace box. It’ll take few seconds to improve your text.
The thin space is used to separate
– numbers from the symbols of the units: 4 cm, 20 min, 25 °C, 75 %
– groups of three digits in large numbers when the comma is not used: 23 500, 2 000 000
– numbers from mathematic operators: 2 + 2 = 4
– numbers, formulae and symbols in chemical reactions: CH4 + 2 O2 → CO2 + 2 H2O
A special feature of the thin space is that it’s non-breakable, meaning that it keeps the members that it’s separating together [oxymoron] preventing them to split at the end of a line. This is important as numbers and units, and the different groups of digits in a number can’t be split. Ever. Well, despite it being incorrect, it is physically possible and people do it; but those who’ve read this post have no excuse for doing it wrong.
MESTRES, J. M. et al. Manual d’estil: La redacció i l’edició de textos. 4a ed. Vic: Eumo Editorial, 2009.
COHEN, E. R. et al. Quantities, Units and Symbols in Physical Chemistry [online]. 3rd ed., 2nd print. Cambridge: IUPAC & RSC Publishing, 2008.