▁regular ▁issue . ▁ ▁Both ▁the ▁st amps ▁and ▁the ▁entire ▁sheet ▁are ▁valid ▁for ▁ma iling , ▁although ▁they ▁are ▁almost ▁always ▁sold ▁above ▁face ▁value ▁and ▁kept ▁in ▁mint ▁collection ▁by ▁collect ors ; ▁a ▁hand ful ▁of ▁us ages ▁may ▁be ▁found ▁as ▁phil atel ic ▁covers . ▁ ▁Luxemb ourg ▁issued ▁the ▁first ▁generally ▁recognized ▁sou venir ▁sheet ▁in ▁ 1 9 2 3 , ▁a ▁single ▁ 1 0 - fran c ▁stamp , ▁not ▁otherwise ▁available , ▁in set ▁in ▁a ▁larger ▁blank ▁sheet . ▁ ▁The ▁purpose ▁was ▁to ▁honor ▁the ▁birth ▁of ▁Princess ▁Elisabeth . ▁ ▁However , ▁this ▁country ▁had ▁produced ▁a ▁somewhat ▁similar ▁issue ▁in ▁ 1 9 2 1 , ▁a ▁small ▁sheet ▁of ▁ 5 ▁st amps ▁to ▁celebr ate ▁the ▁birth ▁of ▁Prince ▁Jean , ▁and ▁an ▁even ▁earlier ▁sheet ▁of ▁ 1 0 ▁st amps ▁for ▁the ▁acc ession ▁of ▁William ▁IV ▁in ▁ 1 9 0 6 . ▁ ▁Ne ither ▁had ▁margin ▁ins cri ptions ▁and ▁both ▁consisted ▁of ▁regular ▁issue ▁st amps ▁that ▁were ▁otherwise ▁available . ▁Since ▁the ▁ 1 9 2 0 s ▁many ▁other ▁countries ▁have ▁produced ▁sou venir ▁sheets . ▁ ▁The ▁United ▁States ▁issued ▁its ▁first ▁one ▁in ▁ 1 9 2 6 , ▁and ▁printed ▁several ▁more ▁in ▁the ▁ 1 9 3 0 s , ▁with ▁new ▁ones ▁occasionally ▁issued ▁since ▁that ▁time . ▁ ▁Starting ▁in ▁ 1 9 5 5 ▁the ▁United ▁Nations ▁has ▁occasionally ▁produced ▁them