how is an SSL certificate chain bundle arranged?

I have 4 certificate files like this:

1.certum_certificate.crt2.certum_certificate.pem3.Intermediate_CA2.cer4.Intermediate_CA.cer5.Root_CA.cer

I put these files content by this order in a bundle file and I figured out that my SSL chain is incomplete.

How should I arrange them in bundle file?