World Guide Numbers (WGN) are listed, when known.  WGN assigned numbers are coded, such as 46-03-01.  The first two numbers are the assigned State number in alphabetical order.  The second two numbers represent the county number, also in alphabetical order.  The third pair of characters or numbers is the bridge number.  All non authentic bridges, as well as a few authentic bridges, are numbered with an alpha character in lieu of the last two digits, i.e., 45-08-A (Vermont's Joe's Pond non authentic Stringer Covered Bridge). 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. Pennsylvania - General information.   The first covered bridge to be built in the United States was located on Market Street in Philadelphia, Pennsylvania, and spanned the Schuylkill River.  It was built in 1805.  It was quite a lengthy structure.  Shortly thereafter, the second covered span in America, a 1,008 foot giant that crossed the Delaware River between Morrisville, Pennsylvania, and Trenton, New Jersey was built.  The greatest period of bridge building activity took place from the 1820s to the end of the nineteenth century. During this time period there were at least 1,500 covered bridges built in a variety of sizes and truss designs. Records show that at one time, all but three of Pennsylvania's sixty-seven counties had covered bridges in use.  Today, there are still forty counties in which covered bridges still stand,   Fortunately interest in them has been growing and there are many people working to preserve them.  A small number have even been disassembled and placed in storage until full restoration can be scheduled.  Pennsylvania not only leads the nation as the birthplace of covered bridges, but it has the largest number of covered bridges still standing.  Of these, 151 still carry motorized vehicular traffic.  Many of those not open to vehicular traffic have been preserved in parks or on private property and are still in use for two-legged and four-legged traffic.