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+By default, the launcher will download and launch the appropriate .jar for the selected Minecraft version, but you can substitute this file with your own.
+
+This allows you to compile in your own .jar mods.
+
+## Getting .jars
+
+You can normally get unmodified game .jars for Minecraft from the `versions/` folder within its main data folder. Remember that if you modify the .jar, you should remove the manifest and signature files from the `META-INF/` folder.
+
+## Using custom .jars
+
+In your modpack's `src/` folder, place your custom .jar inside and name it `custom_jar.jar`. The launcher will automatically use it.
+
+You may want to make the .jar client only (with a `_CLIENT` folder).
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