fig2 a shows a spanwise cross - sectional view of a joint / overlap region 100 of two wing boxes 110 , 120 according to a first embodiment of the invention . the first , inner wing box 110 comprises an n - shaped spar 111 with a generally horizontal top portion 112 and two generally vertical side portions 113 , 114 . the side portions 113 , 114 are each formed with an outwardly extending generally horizontal flange portion 115 , 116 ( giving the spar 111 an omega - shape ). the inner wing box 110 also comprises a lower cover 117 extending underneath and between the outer edges of the flange portions 115 , 116 . the second , outer wing box 120 comprises a generally horizontal upper cover 122 on top of the top portion 112 of the first wing box 110 . the second wing box 120 also comprises two generally vertical outwardly facing side spars 123 , 124 . each side spar 123 , 124 is angled outwards at a shallow draft angle of 3 degrees . each side spar 123 , 124 comprises an upper 125 a , 126 a and lower 125 b , 126 b flange portion extending outwards . the spar flange portions 125 a , 125 b , 126 a , 126 b extend outwards the same distance as the flange portions 115 , 116 of the n - shape spar 111 of the first wing box 110 . the upper cover 122 extends between from slightly beyond the upper flange portions 125 a , 126 a . the second wing box 120 also comprises a generally horizontal lower cover 127 beneath the lower cover 117 of the first wing box 110 . the lower cover 127 extends between from slightly beyond the lower flange portions 125 b , 126 b . fig2 b shows a perspective view of the two wing boxes 110 , 120 in fig2 a . here , it can be seen that shear bolts 130 are used to form the two wing boxes 110 , 120 and attach them together . for example , shear bolts 130 are bolted through the following portions : upper cover 122 of second wing box 120 and upper flange portions 125 a , 126 a side spars 123 , 124 of second wing box 120 , upper cover 122 of second wing box 120 and top portion 112 of n - shape spar 111 of first wing box 110 , spars 123 , 124 of second wing box 120 and sides 113 , 114 of first wing box 110 , lower flange portions 125 b , 126 b of side spars 123 , 124 of second wing box 120 , flange portions 115 , 116 of n - shape spar 111 of first wing box 110 , lower cover 117 of first wing box 110 and lower cover 127 of second wing box 120 , and lower flange portions 125 b , 126 b of side spars 123 , 124 of second wing box 120 and lower cover 127 of second wing box 120 . this figure also shows an offset portion 128 of the lower flange 125 b of the second wing box 120 in the overlap region . the offset portion 128 is raised above the plane of the rest of the flange 125 b so as to give a space 129 for the lower cover 117 of the first wing box 110 and the flange portion 115 of the first wing box 110 . a similar offset portion and space is present for the other lower flange 126 b of the second wing box 120 . fig3 shows a chordwise cross - sectional view of a joint / overlap region 200 of two wing boxes 210 , 220 according to a second embodiment of the invention . the first , inner wing box 210 comprises four - sided box spar 211 with a generally horizontal top portion 211 , two generally vertical side portions 212 , 214 and a generally horizontal bottom portion 213 . the second , outer wing box 220 comprises a generally horizontal upper cover 227 on top of the top portion 211 of the first wing box 210 and extending outwards beyond the edges of the top portion 211 . the second wing box 220 also comprises a u - shaped spar 221 with a generally horizontal bottom portion 222 beneath the lower portion 213 of the first wing box 210 and two generally vertical side portions 223 , 224 adjacent the side portions 212 , 214 of the first wing box 210 . each side portion 223 , 224 is angled outwards at a shallow draft angle of 3 degrees . each side portion 223 , 224 comprises an upper flange portion 225 , 226 extending outwards . the flange portions 225 , 226 extend outwards the same distance as the flange portions upper cover 227 of the second wing box 220 . fig4 shows a chordwise cross - sectional view of a wing box 310 , for use in a third embodiment of the invention . the wing box 310 is especially suited for use as an inner wing box , but could also be used as an outer wing box . the wing box 310 comprises a generally horizontal upper cover 312 and two generally vertical inwardly facing side spars 313 , 314 . each side spar 313 , 314 comprises a generally horizontal upper 315 a , 316 a and a generally horizontal lower flange portion 315 b , 316 b extending inwards . the outer vertical portions of the side spars 313 , 314 are in line with the edges of the upper cover 312 . the upper flange portions 315 a , 316 a are underneath the upper cover 312 . a generally horizontal lower cover 317 is beneath the lower flange portions 315 b , 316 b . the edges of the lower cover 317 may be in line with outer vertical portions of the side spars 313 , 314 . as an alternative , as shown by the dashed lines , the lower cover 127 may have “ grow outs ” 319 such that the lower cover 317 may extend between from slightly beyond the side spars 313 , 314 and upper cover 312 . fig5 shows a plan view of two wing boxes 410 , 420 , ready to be joined together according to a fourth embodiment of the invention . as can be seen , the inner wing box 410 has a cross section that decreases as the wing box extends away from the joining region 400 . it is noted that dashed line 430 a represents one of the largest cross - sections of wing box 410 at a root end of the wing box 410 . conversely , the outer wing box 420 has a cross section that increases as the wing box extends away from the joining region 400 . it is noted that dashed line 430 b represents one of the smallest cross - sections of wing box 420 at a tip end of the wing box 420 . in fig5 , the inner wing box 410 is being slid into position with respect to wing box 420 by sliding it inside the wing box 420 so that cross - section 430 a abuts against corresponding cross - section 430 b . this arrangement removes the need for the side portions ( e . g . side spars or side portions of u - or n - shaped spars ) to be at an angle to the upper / lower covers . this is because the inner wing box can be slid along through the outer wing box instead of being placed up / down into it . in the above embodiments , references to inner wing boxes and outer wing boxes refers to which wing box is located inside the other wing box in the joining / overlap region . inner / outer wing boxes do not refer to the relative position of the wing boxes in relation to the root / tip of the wing itself . during manufacture of the above embodiments , the first , inner wing box is fully assembled with its ribs and systems installed . if the inner wing box has grow outs , as in fig4 , these may be removed . the second , outer wing box is also assembled , however , it is not fully assembled and has its upper cover missing at this stage . the shim / fettle allowance of the two wing boxes is scanned so that one of the wing boxes ( generally the first , inner wing box ) is machined to match the interface with the other wing box ( generally the second , outer wing box ). except for the fig5 embodiment , to join the two wing boxes together , the fist , inner wing box is lowered into position with respect to the second , outer wing box . this process is aided by the shallow draft angle in the side portions / spars of the second , outer wing box . once positioned in place , shear bolts are used to secure the two wing boxes together . stringers ( not shown ) may also be connected together through simple splice plates ( not shown ) between the two wing boxes . these splice plates provide a similar function to the crown fittings in the prior art . however , these splice plates are loaded in shear , rather than tension ( as the crown fittings generally are ). once , this has been done , the missing cover of the second , outer wing box , which has been machined to match the profile of the resulting assembly , is installed and further shear bolts are used to attach the cover . in all of the above embodiments , the join region is intended to be placed at a point on the wing away from areas where required aerodynamic contours are required . this makes assembly simplified . whilst the present invention has been described and illustrated with reference to particular embodiments , it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein . by way of example only , certain possible variations will now be described . in the above embodiments , it is envisaged that the two wing boxes are assembled as much as possible separately prior to being joined together . in particular , it is envisaged that at least one of the wing boxes is assembled with all of its ribs prior to being joined to the other wing box . in addition , systems may also be installed on one or more of the wing boxes . however , different elements of the wing boxes may be assembled into the wing boxes after they have been joined together . for example , ribs in the first , inner wing box may be installed after the first wing box has been joined to the second , outer wing box . if the first , inner wing box has external facing spar flanges , allowance needs to be made for these through joggling or the use of a separate component to ensure adequate load transfer across the joint . as an alternative to the embodiment of fig5 , the inner wing box 410 may instead by slid into the outer wing box 410 from the tip end to the root end , depending on the shape of the final wing required and the required angles of the leading edge and trailing edge of different sections of the wing provided by the wing boxes 410 , 420 . the use of overlapping structures can be used in other places on the aircraft , rather than just joining one wing box to another at a chordwise point along the wing . for example , such a joint could be used between a root wing box and the fuselage , between a wing tip and a wing box structure ( although this is unlikely as the wing tip joint is traditionally a temporary joint ), between a horizontal tailplane and the fuselage , between a horizontal tailplane and the other horizontal tailplane or between a vertical tailplane and the fuselage ( although this is unlikely as vertical tail planes are not traditionally made from box structures ). where in the foregoing description , integers or elements are mentioned which have known , obvious or foreseeable equivalents , then such equivalents are herein incorporated as if individually set forth . reference should be made to the claims for determining the true scope of the present invention , which should be construed so as to encompass any such equivalents . it will also be appreciated by the reader that integers or features of the invention that are described as preferable , advantageous , convenient or the like are optional and do not limit the scope of the independent claims . moreover , it is to be understood that such optional integers or features , whilst of possible benefit in some embodiments of the invention , may not be desirable , and may therefore be absent , in other embodiments .