The symbol for barred spiral galaxies in the Hubble sequence is SB.
The “S” identifies a spiral galaxy, while “B” indicates that a straight or slightly oval bar of stars crosses its central bulge. In the tuning-fork diagram, ordinary spirals occupy the S branch and barred spirals occupy the parallel SB branch. The Milky Way is generally classified as a barred spiral, often written SBc or a similar subtype.
The lowercase letter describes the arms and bulge: SBa or SBb galaxies have more tightly wound arms and prominent bulges, while SBc galaxies have looser arms and smaller bulges. Examples include NGC 1300, Messier 95, and NGC 1365.
A common mix-up is confusing SB with “spiral bulge” or treating it as a single precise galaxy type. SB is the broad barred-spiral designation; added letters and symbols provide finer classifications. The sequence is morphological, not a proven timeline of galaxy evolution.