The Rast scale is a heptatonic scale that can be built using the following steps between notes: 1, ¾, ¾, 1, 1, ¾, ¾.
If we use (b/2) to denote a quarter note flat (a drop by a quarter tone), then one example of the Rast scale would be C, D, E(b/2), F, G, A, B(b/2). This example is shown below in traditional notation (the horizontal line through b denotes a drop by a quarter note) and in guitar tablature notation (to the extent that guitar tablature allows quarter notes).
This is an Arabic scale that uses quarter tone intervals. One way to obtain this scale is to and to drop the third and seventh degrees of the standard major scale by a quarter tone. The example above is the C major scale (C, D, E, F, G, A, B) with E and B dropped by a quarter tone.