Your word proces­sor as­sumes that a word space marks a safe place to flow text onto a new line or page. A non­break­ing space is the same width as a word space, but it pre­vents the text from flow­ing to a new line or page. It’s like in­vis­i­ble glue be­tween the words on ei­ther side.

Put a non­break­ing space be­fore any nu­meric or al­pha­betic ref­er­ence to pre­vent awk­ward breaks. Re­call this ex­am­ple from para­graph and sec­tion marks:

In the top ex­am­ple, nor­mal word spaces come af­ter the § and ¶ sym­bols, and the nu­meric ref­er­ences in­cor­rectly ap­pear on the next line.

In the bot­tom ex­am­ple, non­break­ing spaces come af­ter the § and ¶ sym­bols. This time, the sym­bols and the nu­meric ref­er­ences stay together.

An ex­am­ple ripped from the headlines:

Also use a non­break­ing space af­ter other ab­bre­vi­ated ref­er­ence marks (Ex. A, Fig. 23), hon­orifics and ti­tles (Sgt. Rock, Ms. Mar­vel), and trade­mark and copy­right sym­bols.

Non­break­ing spaces can be in­serted into HTML doc­u­ments ei­ther with an es­cape code ( ) or by typ­ing a non­break­ing space char­ac­ter (us­ing what­ever key short­cut is as­signed to it in your text editor).

If you be­lieve in the prin­ci­ple that source code should be op­ti­mized for read­abil­ity—I do—then you should use the   es­cape code, as it makes the non­break­ing space vis­i­ble and explicit.