Rashi would sometimes derive the meaning of a word from the meaning of its underlying Biblical root. In applying this method Rashi would use all available grammatical methods to study the meanings of related roots. The next paragraph presents one such rule.

There are 1900 Biblical roots. Of these 1900 roots about half involve X-Vav-Y X-Y-Y X-Y-Hey pairs. These roots (with one root letter weak) often, but not always, have related meanings. Consequently, very often, but not always. one can infer the meaning of a X-Y-Y root from the related X-Y-Hey or X-Vav-Y root.

Rashi believed in two grammatical systems. He believed in the traditional tri-literial (3-letter) root methods used to conjugate verbs and taught in all elementary schools.

Besides the conjugational root system Rashi also believed in a semantic root system. This is a separate system that enables derivation of root meaning from other roots.

When dealing with the word Tauv-Caph-Lamed-Tauv, Techelet we see
• A Biblical root of Caph-Lamed which mean the end, finish, completion,
• The prefix and suffix Tauv which literally means sign. In general words with a prefix or suffix Tauv usually indicate a sign, example,resemblance of what follows. Some simple examples of this principle might be the word pairs,
  • Shin-Beth:Tauv-Shin-Beth-Hey; Shov:Teshuva which means return:repentance (a return to ones former state), or
  • Resh-Aleph-Shin:Resh-Aleph-Shin-Yud-Tauv; Rosh:Reshith which means head:beginning, or
  • Pay-Lamed-Lamed:Tauv-Pay-Lamed-Hey; Palel:Tefilah which means judgement:prayer (Prayer is considered self-judgement before God), or
  • Yud-Lamed-Daleth:Tauv-Vav-Lamed-Daleth-Vav-Tauv; Yalad:Toldoth which means birth:genealogies.
  As can be seen the principle of Tauv as meaning sign, example, resemblance can have some vagueness and subjectivity associated with it.

Rashi's comment follows from the above: Tauv-Caph-Lamed-Tauv:Techeleth means sky-blue, a resemblance of the end of the day. Here the Tauv indicates resemblance while Caph-Lamed means end, completion.

In deriving this etymology we don't mean to take sides on all the controversies that exist on the exact meaning of Techeleth. The basic meaning is that Techelet refers to a sky-blue.
• Rashi posits that Techelet refers to the dark blue of dusk (the end of day)
• Rambam posits that Techelet refers to the blue of midday, a resemblance to the colors of the end of the day
• Rav Hirsch suggests that Techeleth refers to the midday blue of the horizon. Here horizon refers to the end of the field of vision.

I have brought these controversies to show how an underlying Rashi method, meaning of a root and prefix letter, while shedding light on a word can still allow for controversies and differences of opinion.

Advanced Rashi: We have ignored the double Tauv of Techeleth, which has both a prefix and suffix tauv. Perhaps some advanced study can shed more light on the above.