A layer of membrane called the envelope (some but not all viruses)
In addition to the capsid, some viruses also have an external lipid membrane known as an envelope, which surrounds the entire capsid.
Viruses with envelopes do not provide instructions for the envelope lipids. Instead, they "borrow" a patch from the host membranes on their way out of the cell. Envelopes do, however, contain proteins that are specified by the virus, which often help viral particles bind to host cells.