Difference between ERRNO 19 and 20 in OpenSSL

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http://serverfault.com – I have a valid certificate installed on a server and when using the -CApath option with OpenSSL, the certificate chain is valid. However if I don't provide the -CApath option I receive this: 19:self signed certificate in certificate chain When I try Google's HTTPS port without -CApath, OpenSSL returns: 20:unable to get local issuer certificate What's the difference between the two? (HowTos)