An extension ladder comprising: a base section having a right base rail and a left base rail in parallel and spaced relation with the right base rail, the right base rail having a C-shaped cross-section formed by a right base flange attached to a base web and a left base flange attached to the base web with the base web disposed between the right base flange and the left base flange, the base section having base rungs attached to the right and left base rails and disposed in front of and outside of the right base flange of the right base rail; a fly section having a right fly rail and a left fly rail in parallel and spaced relation with the right fly rail, the fly section having fly rungs attached to and extending in between the right and left fly rails, the right base flange and the left base flange disposed about the right fly rail with the right fly rail disposed in between the right and left base flanges, the fly section nested in the base section and configured for at least a portion of the fly section to slide up above the base section and for the fly section to slide back down relative to the base section while remaining engaged with the base section through the right and left base rails disposed about the right and left fly rails, respectively, the right fly rail formed by a right fly flange attached to a fly web and a left fly flange attached to the fly web with the fly web disposed between the right fly flange and the left fly flange, the right fly rail having a top and a bottom, the top is part of the portion that is configured to slide up above the base section; a cap attached to the top, the cap having a surface which extends between the right fly flange and the fly web and the left fly flange; and a locking mechanism to fix and lock the fly section to the base section at a desired position relative to the base section, wherein the base rails (14, 16) and the fly rails (28, 30) are straight, characterised in that the locking mechanism is a J-lock that extends from a side of a rung of the base section and back through a lock hole in the right base rail and into a rung of the fly section to lock the fly section and the base section together, the fly section configured to slide relative to the base section when the J-lock is pulled out from the rung of the fly section; in that the base rungs comprise a tapered portion at either end having a flattened end that fits against and contacts and is in parallel with the respective base flange; and in that the base rail comprises a J-lock rung having a squared or flat face from which the J-lock extends. a base section having a right base rail and a left base rail in parallel and spaced relation with the right base rail, the right base rail having a C-shaped cross-section formed by a right base flange attached to a base web and a left base flange attached to the base web with the base web disposed between the right base flange and the left base flange, the base section having base rungs attached to the right and left base rails and disposed in front of and outside of the right base flange of the right base rail; a fly section having a right fly rail and a left fly rail in parallel and spaced relation with the right fly rail, the fly section having fly rungs attached to and extending in between the right and left fly rails, the right base flange and the left base flange disposed about the right fly rail with the right fly rail disposed in between the right and left base flanges, the fly section nested in the base section and configured for at least a portion of the fly section to slide up above the base section and for the fly section to slide back down relative to the base section while remaining engaged with the base section through the right and left base rails disposed about the right and left fly rails, respectively, the right fly rail formed by a right fly flange attached to a fly web and a left fly flange attached to the fly web with the fly web disposed between the right fly flange and the left fly flange, the right fly rail having a top and a bottom, the top is part of the portion that is configured to slide up above the base section; a cap attached to the top, the cap having a surface which extends between the right fly flange and the fly web and the left fly flange; and a locking mechanism to fix and lock the fly section to the base section at a desired position relative to the base section, wherein the base rails (14, 16) and the fly rails (28, 30) are straight, characterised in that the locking mechanism is a J-lock that extends from a side of a rung of the base section and back through a lock hole in the right base rail and into a rung of the fly section to lock the fly section and the base section together, the fly section configured to slide relative to the base section when the J-lock is pulled out from the rung of the fly section; in that the base rungs comprise a tapered portion at either end having a flattened end that fits against and contacts and is in parallel with the respective base flange; and in that the base rail comprises a J-lock rung having a squared or flat face from which the J-lock extends.