Mathematical variables represented by Latin and lower-case Greek letters are usually displayed in italic. Vectors are often displayed in upright boldface. Word 2007 allows the user to control and change these choices with the italic and bold formatting buttons on the Home tab of the user-interface ribbon or with the italic and bold hot keys (Ctrl+i and Ctrl+b). A basic principal is that in a math zone letters are the only characters that are affected by these attributes. Operators like + and = and delimiters like  are not changed by the states of the italic and bold buttons. Such characters are always rendered as upright, normal-weight characters in a math zone.
Let’s start with italic letters. If the italic button is on, as it normally is in a math zone, and you type a Latin letter (A-Z or a-z) or lower-case Greek letter like \beta (β), the letter is converted to the corresponding Unicode math italic letter. These special characters have the correct spacing for mathematical variables and may even have different glyphs from ordinary italic letters. The Unicode math italic A-z codes are in the range U+1D434..U+1D467, except for math italic h. Since the math italic h already existed when the other math italics were added, it retains its original code of U+210E. The math italic lower case Greek letters are in the range U+1D6FC..U+1D71B. Internally in Windows applications these characters are represented by Unicode surrogate pairs (see Sec. 3.8 of the Unicode Standard 5.0).
You can see the code for a character by typing Alt+x immediately after the character. This hot key replaces the character by its code. Typing Ctrl+z undoes this transformation or you can type Alt+x again, since Alt+x toggles between code and character. For example, if you type Alt+x after a math italic b, the b will be replaced by its code 1D44F. Typing Alt+x again, replaces the 1D44F by the math italic b.
If you want to convert math italic characters to upright (nonitalic) characters, select them and click the italic button to the off state or type Ctrl+i, which toggles the italic state to off. Clicking it back to the on state (or typing another Ctrl+i) converts the characters back to math italic. You can also get upright characters by enclosing them in double quotes. For example, a_”coh” builds up to a math italic a with the upright subscript coh, which might be an abbreviation for “coherent”. Such an abbreviation doesn’t represent the product of the letters c, o, and h, and hence should be displayed with upright letters rather than math italic.
The bold button and Ctrl+b hot key work similarly to the italic button and Ctrl+i hot key, respectively, in toggling the math bold attribute for Latin and Greek letters. For regular Roman-style characters you can use these buttons and hot keys to switch between upright, italic, bold and bold italic. You can use the bold button and hot key to switch between Fraktur and bold Fraktur and between Script and bold Script.
The italic and bold attribute states are persisted in the OMML file format. When OMML is embedded in comments in HTML, the HTML tags <i> and <b> are used. When OMML is embedded in the Word docx or rtf formats, the <m:sty> tag or \msty N control word, respectively, is used. Readers of these formats need to take the settings into account both in the text run character formatting as well as in the appropriate choice of upright or math alphabetic characters.
Although the use of italic and bold in math zones differs from that in normal text, it feels quite natural and it facilitates editing these properties of mathematical variables.