I don't doubt that Michel has got it right, and I'm sure his ref's will give you full information; however, I just wanted to add that in tour case I think this is referring, NOT to 'cards' as such, but rather, your 'tiroir' is a withdrawable rack unit [as per your ealier question...]

19" racks are dicvided up height-wise into 'units', colloquially referred to as 'U' which equal 1.75" of height --- so 6U means it needs 10.5" (approx. 266.7 mm) of rack space -- but the measurement in either inches or mm is spurious, since racks are made and measured in 'U'

As for the 'T', I can't suggest anything better than Michel's idea --- it refers here to the pitch or spacing [presumably 'Thickness'?] of the cards --- however, I don't see much sense in suggesting a width of 9 of these module widths, unless of course this is not taking up the full rack width --- the racks are referred to as 19" because that is there overall width (pitch or spacing), although the actual internal width (= usable space) is much nearer 17"

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Note added at 25 mins (2005-01-16 18:29:24 GMT)
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Looking again at Michel\'s answer, I see that he suggests 1 T = 0.2\" --- this is likely to be the pitch of the edge-connector used on the bakc of the board to plug it into the rack frame with; I just don\'t see how this works in terms of your \'encombrement\', as giving a pitch of 1.8\" doesn\'t make a lot of sense to me --- though heaven knows, sometimes they DO have some odd ways of expressing things!

Maybe you have a bit more surrounding context, the sizes of other modules etc. that might help to pin this one down?