What are the standard substance weights for envelopes?

• 16 lb. Used for overseas air mail envelopes.
• 20 lb. Used for commercial envelopes where strength and opacity is not a factor.
• 24 lb. Used for most open side official and commercial envelopes.
• 28 lb. Used for most open and catalog envelopes and large (i.e. 11, 12 and 14) size commercial envelopes.
• 32 lb. Used for heavy duty envelopes and clasp envelopes.
• 40 lb. Used for large envelopes, heavy duty envelopes or envelopes used for storage and frequent reference.