work was that the length of key and the length of time thought necessary to break
the wheels were divided by about 2 and 4 respectively.
At the same time research on key-breaking for X2oneback limitation was
begun, and after some months of evolution the method reached its final form as
described in 26B(b),26E.
(c) The first de-chis
When the X2oneback + P5twoback limitation was reintroduced in mid-December,
1943 it was no longer possible with the equipment of that time to set the motors
and psis mechanically, and at the same time the main source of decodes and the
whole source of employment for the Testery, dried up, since depths could no longer
occur. So the Testery had to master the art of setting the psis by hand from the
de-chis prepared by the Newmanry, and this became their main job. But, more
important, the month's wheel patterns could no longer be broken from depth, and
the task of breaking the wheels from Z had to be attempted. The Bream chis for
January, 1944 were broken within the first fortnight with the comparatively
primitive equipment of the time - Colossus I (see 52(c) was not yet in action.
Two messages on the same QEP were set on the chis and de-chied. From these two
de-chis, by the method of applying the psis obtained from a break in one to the
other de-chi (see 28C(a)) the psi patterns were obtained within an hour. The Mu37
proved to have 26 dots which helps to explain this remarkably short time.
The breaking of the February Bream chis was greatly helped by the use of
Colossus I. No pairs of messages on the same QEP were available, and attempts by
the Testery to break the psis from the de-chis sent over were at first fruitless.
Finally however the psis were broken with great difficulty and effort from one de-
chi. The Mu37 dottage proved to be only 19, which explained the difficulty
encountered. It was now evident that the problem of the X2oneback + P5twoback
limitation had been mastered in both sections. A detailed account of psi-breaking
from de-chi in given in 28C.
(d) The X2oneback + PSI'1oneback + P5twoback limitation
This triple limitation first appeared in June, 1944 on Codfish and Gurnard.
Its action is described in 11B(g)iv); its stay was brief as in December, 1944 the
Germans began taking the P5twoback component out of the limitation on the various
links; thus it gave rise to the X2oneback + PSI'1oneback limitation (see B(g)(ii),
11B(i)), which became the standard limitation on the majority of links, the
remainder reverting to the old X2oneback limitation. It did not cause any new
difficulties apart from slightly complicating the process of de-chi breaking.
(e) Daily Change
The introduction of daily change of all wheel patterns in the summer of 1944 meant
that the time and energy previously expended to release a whole month's traffic
for setting now only released one day's traffic. The emphasis in the Testery as
well as the Newmanry changed from wheelsetting to the much more difficult job of
wheelbreaking. But the concerted efforts of both sections met with such success
that the production figures for August, 1944, the first month with daily change on
all links, was higher than ever before, and the figures continued to rise steadily
month after month.