this disclosure , its aspects and implementations , are not limited to the specific components or assembly procedures disclosed herein . many additional components and assembly procedures known in the art consistent with the intended swimming aids and / or assembly procedures for swimming aid will become apparent for use with implementations of swimming aids in this disclosure . accordingly , for example , although particular swimming aids are disclosed , such swimming aids and implementing components may comprise any shape , size , style , type , model , version , measurement , concentration , material , quantity , and / or the like as is known in the art for such swimming aids and implementing components , consistent with the intended operation of swimming aids . particular embodiments of the swim fins in this disclosure may be used to keep both feet as close together for use as a training device for swimmers . embodiments may be easily attached and detached to separate the fins , may be relatively easy and inexpensive to manufacture and may be modified and situated to accommodate various feet sizes of the user . particular embodiments are relatively lightweight , durable and waterproof . the disclosure generally relates to methods and structure for detachably coupling two swim fins . to promote longevity , particular implementations of the disclosure rely upon a more durable coupling device that has a center locking mechanism . the locking mechanism is harder and includes more plastic than the remainder of the composition of the fins . the locking mechanism has a hardness durometer scale measurement rating of at least about 90 d ( hereafter “ hard ” or “ harder ”) whereas the foot pocket and balance of the fins further from the locking mechanism has a hardness durometer scale measurement rating of less than about 60 d . the coupling devices described in the first particular embodiment of fig1 - 4 employ a hard center locking mechanism that can removably slide between grooves in the fins , sometimes referred to as a sliding key and groove coupling . other locking mechanisms included in the disclosure may be described as peg and groove , latch and catch , zipper , opposite polar magnets , key and lock , plate and recess , and disk and slot . other compositions of coupling devices , such as using a metal or plastic zipper slider and teeth of corresponding materials are also included in this disclosure . swim fins of embodiments of this disclosure are composed of resilient material to allow for coupling and use in unison without failure due to force exerted upon the coupling . the central locking device , whether it is by sliding a locking key with corresponding grooves in the fins , peg and groove mechanism , latch and catch mechanism , zipper mechanism , opposite polar magnets with one polar magnet installed on the inner edge of each fin , sliding groove mechanism , plate and recess mechanism or disk and slot mechanism , is harder and uses more plastic with a durometer reading of at least about 90 d , as compared with the remaining composition of the foot pockets and the balance of the fins that may be made with plastic having a durometer reading of about 60 d or less . the process for making the center locking structure harder involves making the center lock first . the center locking mechanism is made as a mold material and is injected into a mold and then allowed to harden . the center locking mechanism area may also comprise finger - like channels and reinforcing ribs throughout for added rigidity and strength , which do not appear in the remainder of the fins . the softer , more rubber - like material with a lower durometer rating is injected into the mold around the center locking mechanism . the softer , more rubber - like material is injected around the harder center locking mechanism , which spreads into the finger - like channels to assist in creating a strong connection between the harder center locking mechanism and the body of each fin . this process will be clear to one of ordinary skill in the art of forming molded plastic and rubber components from this disclosure . in fig1 , the first fin , or right foot fin 101 , has at least two rectangular slots 103 that are longitudinally spaced on a lateral side nearest to the second fin , or left foot fin 102 . the left fin has at least two slightly larger rectangular pegs 104 that are complementally spaced on a lateral side nearest to the right fin . when the rectangular pegs of the left fin are angled and then press fitted into the at least two slots on the right fin , they form a monofin which can later be detached . when attached , the monofin is sufficiently strong to allow a wearer to use the monofin for swimming . in fig2 , the right fin 101 has at least two keepers 105 that each have a permanently affixed first base . one edge of the base is backwardly curved at an edge nearest to the left fin 102 . the at least two first bases on the right fin are longitudinally spaced on a side nearest to the left fin 102 . the left fin 102 has at least two latches 106 each with a permanently affixed second base and each pivotally connected to rotatable lever 108 which can be raised and lowered over a range of about 180 degrees in order to extend a ring 107 which is also attached to the second base to catch on the backwardly curved edge of the affixed first base 105 on the right fin 101 . the at least two latches 106 are complementally located on a lateral side nearest to the right fin 101 . the latches and keepers can be closed on each other to detachable couple both fins . when closed , they form a monofin . in fig3 , the right fin 101 has a zipper slider 108 mounted on a permanently affixed row of teeth on a side nearest to the left fin 102 . the left fin 102 has a correspondingly , permanently affixed row of teeth on the side nearest to the right fin 101 . when the zipper slider 108 is moved upwardly from the toe of the right fin , the fins are detachably joined . the monofin is sufficiently strong to allow use as a swimming device . it is further anticipated that the zipper slider may be made of metal , plastic , nylon or other material common to zippers , and the teeth will also be made of similar materials . in fig4 , the right fin 101 has a longitudinal groove on the side nearest the left fin 102 , and the left fin has a correspondingly longitudinal groove on a side nearest the right fin . a rectangular key 111 with protruding lips 112 on opposing sides can be inserted in the corresponding grooves , to detachably couple both fins . when coupled , the wearer can use the monofin as a swimming device . in fig5 , another embodiment of right foot fin 201 and left foot fin 202 of a monofin pair 200 are shown . in this embodiment , both the left foot fin 201 and the right foot fin 202 are identical so that each includes not only the same parts , but both include the same parts in the same locations so that they are not mirror images of each other , as is typical with conventional fins , but are the same and could even be formed in the same mold . as illustrated , both the right foot fin 201 and the left foot fin 202 include a left side heel connector 203 and a right side heel connector receiver 204 , a fin blade edge 205 and a fin blade groove or receiver 206 . although particular connectors 203 , 204 for the heels , and particular connectors 205 , 206 for the fin blade are shown , it is contemplated that any style of connector may be used or substituted for those shown in the figures , and in particular any of the other styles of connectors disclosed elsewhere in this disclosure . in particular , the left side heel connector 203 and the right side heel connector 204 are designed to mate with each other to hold the heel of the right foot fin 201 to the heel of the left foot fin 202 . it is not critical whether the receiving side of that connector pair is on the left side or right side of the heel , but it is useful to have the connectors placed adjacent to the location where the wearer &# 39 ; s heels will be positioned when wearing the fins . the fin blade edge 205 and fin blade groove or receiver 206 are shown in fig5 with the edge 205 portion of the connector on the left side of the fin blade , and the groove or receiver 206 angled right to left on the surface of the fin right of center . these connector parts can also be swapped in their respective positioning and angles with equal effectiveness ( by placing the fin blade edge 205 portion of the connector on the right side of the fin blade and the groove or receiver 206 angled left to right on the surface of the fin left of center ). as illustrated in fig6 , by joining the heel connectors 203 , 204 together , and joining the fin blade connectors 205 , 206 together , the two separate fins form a monofin . different from previous embodiments , however , this monofin is formed of two fins that are not unique to a left or right foot and can be interchanged , and allows the user &# 39 ; s fins to overlap which separates the wearer &# 39 ; s heels a little and allows the wearer &# 39 ; s toes to angle toward each other for comfort when using a dolphin kick style of swimming . like with the materials used for forming the locking mechanisms of previous embodiments , the locking mechanisms , or connectors , of the embodiments of fig5 - 7 similarly may be formed of materials having a hardness durometer scale measurement rating of at least about 90 d ( hereafter “ hard ” or “ harder ”) whereas the foot pocket and balance of the fins further from the locking mechanisms , or connectors , may have a hardness durometer scale measurement rating of less than about 60 d . use of a harder material for forming the locking mechanisms or connectors allows the body and pocket of the fins to be comfortable while allowing the locking mechanisms , or connectors , to maintain the connection between the left and right fins even if the wearer is not using a uniform dolphin kick so that the wearer does not unintentionally disconnect the two fins from each other . also , as with all of the embodiments of this disclosure , the fins are effective as separate fins 201 , 202 as well as being effective as a monofin pair 200 when combined . fig7 illustrates another particular embodiment of a single fin 210 of a monofin pair comprising left and right heel connectors 203 , 204 and both an extending 212 and a receiving 211 fin blade connectors . in use , rather than inserting a fin blade edge into a groove or receiver on the corresponding fin blade of the monofin pair 200 as in the embodiment of fig5 and 6 , the extending fin blade connector 212 may be received into the receiving fin blade connector 211 on a corresponding fin blade of a monofin pair . in the particular embodiment of fig7 , the fin blade connector 212 is hook shaped and the fin blade connector receiver 211 is an aperture extending through the fin blade . those of ordinary skill in the art will readily be aware of other forms and styles of connectors suitable for various applications from this disclosure .