>I created a header file called CONSTANTS.H to define the CRLF constant with the CHR(13) + CHR(10) (CARRIAGE RETURN LINE FEED) and placed it in my development environment and also added it to my project in the Text Files area.
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>I did a #INCLUDE in the MAIN.PRG program to include the CONSTANTS.H file. However, when I try to use the CRLF in a form, I get an error telling me that that variable doesn't exist.
>
>Have I set this up wrong? (Obviously!)

You've to include your header file in every PRG, form and class that refereneces constants from it. For Form/Class go to Form/Class menu, select Include File and add include file there.