The TEXTMETRIC structure contains basic information about a physical font. All sizes are specified in logical units; that is, they depend on the current mapping mode of the display context.

typedef struct tagTEXTMETRIC {
LONG tmHeight;
LONG tmAscent;
LONG tmDescent;
LONG tmInternalLeading;
LONG tmExternalLeading;
LONG tmAveCharWidth;
LONG tmMaxCharWidth;
LONG tmWeight;
LONG tmOverhang;
LONG tmDigitizedAspectX;
LONG tmDigitizedAspectY;
TCHAR tmFirstChar;
TCHAR tmLastChar;
TCHAR tmDefaultChar;
TCHAR tmBreakChar;
BYTE tmItalic;
BYTE tmUnderlined;
BYTE tmStruckOut;
BYTE tmPitchAndFamily;
BYTE tmCharSet;
} TEXTMETRIC, *PTEXTMETRIC;
Members
tmHeight
Specifies the height (ascent + descent) of characters.
tmAscent
Specifies the ascent (units above the base line) of characters.
tmDescent
Specifies the descent (units below the base line) of characters.
tmInternalLeading
Specifies the amount of leading (space) inside the bounds set by the tmHeight member. Accent marks and other diacritical characters may occur in this area. The designer may set this member to zero.
tmExternalLeading
Specifies the amount of extra leading (space) that the application adds between rows. Since this area is outside the font, it contains no marks and is not altered by text output calls in either OPAQUE or TRANSPARENT mode. The designer may set this member to zero.
tmAveCharWidth
Specifies the average width of characters in the font (generally defined as the width of the letter x). This value does not include the overhang required for bold or italic characters.
tmMaxCharWidth
Specifies the width of the widest character in the font.
tmWeight
Specifies the weight of the font.
tmOverhang
Specifies the extra width per string that may be added to some synthesized fonts. When synthesizing some attributes, such as bold or italic, graphics device interface (GDI) or a device may have to add width to a string on both a per-character and per-string basis. For example, GDI makes a string bold by expanding the spacing of each character and overstriking by an offset value; it italicizes a font by shearing the string. In either case, there is an overhang past the basic string. For bold strings, the overhang is the distance by which the overstrike is offset. For italic strings, the overhang is the amount the top of the font is sheared past the bottom of the font.
The tmOverhang member enables the application to determine how much of the character width returned by a GetTextExtentPoint32 function call on a single character is the actual character width and how much is the per-string extra width. The actual width is the extent minus the overhang.

tmDigitizedAspectX
Specifies the horizontal aspect of the device for which the font was designed.
tmDigitizedAspectY
Specifies the vertical aspect of the device for which the font was designed. The ratio of the tmDigitizedAspectX and tmDigitizedAspectY members is the aspect ratio of the device for which the font was designed.
tmFirstChar
Specifies the value of the first character defined in the font.
tmLastChar
Specifies the value of the last character defined in the font.
tmDefaultChar
Specifies the value of the character to be substituted for characters not in the font.
tmBreakChar
Specifies the value of the character that will be used to define word breaks for text justification.
tmItalic
Specifies an italic font if it is nonzero.
tmUnderlined
Specifies an underlined font if it is nonzero.
tmStruckOut
Specifies a strikeout font if it is nonzero.
tmPitchAndFamily
Specifies information about the pitch, the technology, and the family of a physical font.
The four low-order bits of this member specify information about the pitch and the technology of the font. A constant is defined for each of the four bits. Constant Meaning
TMPF_FIXED_PITCH If this bit is set the font is a variable pitch font. If this bit is clear the font is a fixed pitch font. Note very carefully that those meanings are the opposite of what the constant name implies.
TMPF_VECTOR If this bit is set the font is a vector font.
TMPF_TRUETYPE If this bit is set the font is a TrueType font.
TMPF_DEVICE If this bit is set the font is a device font.

An application should carefully test for qualities encoded in these low-order bits, making no arbitrary assumptions. For example, besides having their own bits set, TrueType and PostScript fonts set the TMPF_VECTOR bit. A monospace bitmap font has all of these low-order bits clear; a proportional bitmap font sets the TMPF_FIXED_PITCH bit. A Postscript printer device font sets the TMPF_DEVICE, TMPF_VECTOR, and TMPF_FIXED_PITCH bits.

The four high-order bits of tmPitchAndFamily designate the font's font family. An application can use the value 0xF0 and the bitwise AND operator to mask out the four low-order bits of tmPitchAndFamily, thus obtaining a value that can be directly compared with font family names to find an identical match. For information about font families, see the description of the LOGFONT structure.

tmCharSet
Specifies the character set of the font. The character set can be one of the following values.
ANSI_CHARSET
DEFAULT_CHARSET
SYMBOL_CHARSET
SHIFTJIS_CHARSET
HANGUL_CHARSET
GB2312_CHARSET
CHINESEBIG5_CHARSET
OEM_CHARSET
JOHAB_CHARSET
HEBREW_CHARSET
ARABIC_CHARSET
GREEK_CHARSET
TURKISH_CHARSET
VIETNAMESE_CHARSET
THAI_CHARSET
EASTEUROPE_CHARSET
RUSSIAN_CHARSET
MAC_CHARSET
BALTIC_CHARSET

取自MSDN