Specify an Alternative Default Font

ActiveX and VBA Developer's Guide > Create and Edit AutoCAD Entities > Add Text to Drawings > Substitute Fonts >

If your drawing specifies a font that is not currently on your system, AutoCAD automatically substitutes the font designated as your alternate font. By default, AutoCAD uses the simplex.shx file. However, you can specify a different font if necessary. Use the AltFontFile property on the Preferences object to set the alternative font file name.

If you use a text style that uses a Big Font, you can map it to another font using the AltFontFile property. This system variable uses a default font file pair of txt.shx, bigfont.shx.

If AutoCAD cannot find a font file when a drawing is opened, it applies a default set of font substitution rules.