reference will now be made in detail to various embodiments of the present invention ( s ), examples of which are illustrated in the accompanying drawings and described below . while the invention ( s ) will be described in conjunction with exemplary embodiments , it will be understood that present description is not intended to limit the invention ( s ) to those exemplary embodiments . on the contrary , the invention ( s ) is / are intended to cover not only the exemplary embodiments , but also various alternatives , modifications , equivalents and other embodiments , which may be included within the spirit and scope of the invention as defined by the appended claims . referring to fig1 to fig9 , a multi - stage automatic transmission according to various embodiments of the present invention includes an input shaft 10 , an output shaft 20 , a first planet gear mechanism pg 1 , a second planet gear mechanism pg 2 , a plurality of pairs of external gears , a plurality of power engaging / disengaging members and a restraint member . here , the output shaft 20 is disposed parallel to the input shaft 10 . the first planet gear mechanism pg 1 is coaxially disposed on the input shaft 10 , and has two rotary elements which are connected to the input shaft 10 so as to engage with and disengage from the input shaft 10 . the second planet gear mechanism pg 2 is connected to the first planet gear mechanism pg 1 via the pairs of external gears , and is connected to the output shaft 20 . the plurality of pairs of external gears can transfer power between one rotary element of the rotary elements of the second planet gear mechanism pg 2 that is connected to neither the first planet gear mechanism pg 1 nor the output shaft 20 and the input shaft 10 . the plurality of power engaging / disengaging members can switch power transmission via the pairs of external gears between the rotary elements of the second planet mechanism pg 2 and the input shaft 10 . the restraint member can halt the movement of one rotary element of the rotary elements of the second planet gear mechanism pg 2 which is connected to neither the first planet gear mechanism pg 1 nor the output shaft 20 . the multi - stage automatic transmission which realizes nine forward gears and one rearward gear is constructed of a combination of two planet gear mechanisms , five pairs of external gears and six friction elements which are disposed between the parallel input and output shafts 10 and 20 . here , the friction element is a term that comprehensively includes the power engaging / disengaging member and the restraint member . according to various embodiments , one rotary element of the first planet gear mechanism pg 1 that is connected to the input shaft 10 such that the rotary element can engage with and disengage from the input shaft 10 is connected to the rotary element of the second planet gear mechanism pg 2 via a pair of external gears 51 and 52 . a first brake c 5 which can halt the rotation of the rotary elements connected to the first pair of external gears 51 and 52 is also provided . the first planet gear mechanism pg 1 is configured as a double pinion planet gear mechanism , in which a first linear gear 41 and a first carrier 43 are connected to the input shaft 10 such that they can engage with and disengage from the input shaft 10 . the first carrier 43 is connected to the input shaft 10 via a first clutch c 1 such that it can engage with and disengage from the input shaft 10 . the first linear gear 41 is connected to the input shaft 10 via a second clutch c 2 such that it can engage with and disengage from the input shaft 10 . in addition , the second planet gear mechanism pg 2 is configured as a single pinion planet gear mechanism . a second carrier 102 of the second planet gear mechanism pg 2 is connected to the first carrier 43 of the first planet gear mechanism pg 1 via the first pair of external gears 51 and 52 . a second ring gear 103 of the second planet gear mechanism pg 2 is connected to the output shaft 20 , and is connected to a first ring gear 42 of the first planet gear mechanism pg 1 via a second pair of external gears 61 and 62 . one rotary element of the rotary elements of the second planet gear mechanism pg 2 which is connected to neither the first planet gear mechanism pg 1 nor the output shaft 20 is a second linear gear 101 . the pairs of external gears between the second linear gear 101 and the input shaft 10 include a third pair of external gears 71 and 72 which are linearly disposed between the input shaft 10 and the second linear gear 101 , a fourth pair of external gears 81 and 82 , and a fifth pair of external gears 91 and 92 which are disposed parallel to the third pair of external gears 71 and 72 and the fourth pair of external gears 81 and 82 . the above described configuration is common to various embodiments of the present invention . with reference to fig1 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the input shaft 10 and the third pair of external gears 71 and 72 and a fourth clutch c 4 which is disposed between the input shaft 10 and the fifth pair of external gears 91 and 92 . the restraint members are configured as a second brake c 6 which is directly coupled to the second linear gear 101 . with reference to fig4 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the input shaft 10 and the third pair of external gears 71 and 72 and a fourth clutch c 4 which is disposed between the fifth pair of external gears 91 and 92 and the second linear gear 101 . the restraint members are configured as a second brake c 6 which is directly coupled to the second linear gear 101 . with reference to fig5 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the fourth pair of external gears 81 and 82 and the second linear gear 101 and a fourth clutch c 4 which is disposed between the input shaft 10 and the fifth pair of external gears 91 and 92 . the restraint members are configured as a second brake c 6 which is directly coupled to the second linear gear 101 . with reference to fig6 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the fourth pair of external gears 81 and 82 and the second linear gear 101 and a fourth clutch c 4 which is disposed between the fifth pair of external gears 91 and 92 and the second linear gear 101 . the restraint members are configured as a second brake c 6 which is directly coupled to the second linear gear 101 . in addition , the first brake c 5 can be connected to the first carrier 43 of the first planet gear mechanism pg 1 which is connected to the first pair of external gears 51 and 52 . this configuration is commonly provided in various embodiments except for that shown in fig7 . with reference to fig7 , the first brake c 5 may be connected to the second carrier 102 of the second planet gear mechanism pg 2 which is connected to the first pair of external gears 51 and 52 . that is , the first brake c 5 can be configured such that it is connectable to any element which is connected to the pair of external gears 51 and 52 so as to transfer power . with reference to fig8 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the input shaft 10 and the third pair of external gears 71 and 72 and a fourth clutch c 4 which is disposed between the input shaft 10 and the fifth pair of external gears 91 and 92 . the restraint members are configured as a second brake c 6 which is directly connected between the fourth clutch c 4 and the fifth pair of external gears 91 and 92 . with reference to fig9 , the power engaging / disengaging members may include a third clutch c 3 which is disposed between the input shaft 10 and the third pair of external gears 71 and 72 and a fourth clutch c 4 which is disposed between the input shaft 10 and the fifth pair of external gears 91 and 92 . the restraint members are configured as a second brake c 6 which is directly connected between the third clutch c 3 and the third pair of external gears 71 and 72 . the various embodiments discussed herein , which are configured as above , may realize first to ninth forward shift gears and one rearward gear depending on the operation mode table shown in fig2 . as apparent from the operation mode table , the operating condition for disabling one friction element and enabling the other friction element during sequential shifting to the adjacent gear is satisfied , and the shift ranges between adjacent gears are set to be suitable . for reference , each block in which one of numbers 4 to 10 is written as shown in the figures presents a section which corresponds to one lever in the lever diagram in fig3 . for convenience in explanation and accurate definition in the appended claims , the terms forward or rearward , and etc . are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures . the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teachings . the exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application , to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention , as well as various alternatives and modifications thereof it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents .