World Guide Covered Bridge (WGCB) numbers have been assigned by the National Society for the Preservation of Covered Bridges (NSPCB) are referenced for all listed bridges.  Assigned numbers are coded, such as NH-05-12 or VT-115-a.  The first two characters are the postal abbreviations for the state (formerly digits starting at 01 represented the state in alphabetical order).  The second two digits represents the county in alphabetical order starting at 01 for each state.  The third pair of characters represents the bridges place in the NSPCB tally of that county's bridges, starting at 01 for authentic truss type bridges or a letter for non authentic type bridges, an upper case letter for motor vehicle bridges, and a lower case letter for footbridges.  Additionally, bridges that have a leading "(" before the last letter, are considered O.C.B.I. - "Of Covered Bridge Interest" - a Covered Bridge that is under 12' or is just on the ground and not over anything, i.e., 45-12-(b.

New Hampshire - General information. The first covered bridge did not appear in New Hampshire until the 1820s. Nearly four hundred covered bridges stood in the state at one time. Today, only fifty four of the historic structures can still be found. In addition to these historic covered bridges, there are eight that have been built in the later part of the twentieth century and two in the twenty-first century. Three of them are interstate bridges linking New Hampshire and Vermont crossing the Connecticut River. These bridges, technically belong to the State of New Hampshire, since New Hampshire's western boundary extends to the west shoreline of the Connecticut River. All three were originally toll bridges. At one time a total of thirty-five covered bridges, both highway and railroad, spanned the Connecticut River from the northern extremities in the town of Pittsburg, New Hampshire, to the southern borders of New Hampshire and Vermont where the river enters Massachusetts. Not all of these bridges were interstate given several of them crossed the Connecticut River where it originates entirely within the northern tip of New Hampshire, before it became the state border with Vermont. Records indicate that the earliest bridge, not covered, across the Connecticut River dates back to 1785, when a span was built for Walpole, New Hampshire, across the river to Bellows Falls, Vermont.