Tunisian simple stitch and variants all work the front vertical bar of the stitch.
Tunisian Simple Stitch (Tss)
Insert hook behind front vertical bar. YO and draw up a loop. Tunisian Simple StitchTunisian Simple Stitch
Modified Tunisian Simple Stitch (Tmss)
Tmss gives a similar look to Tss, but it creates a flatter fabric. Insert hook behind front vertical bar AND under front horizontal bar. YO and draw up a loop. Modified Tunisian Simple StitchModified Tunisian Simple Stitch
Back Bar Tunisian Simple Stitch (Tbss)
Insert hook behind back vertical bar only. YO and draw up a loop. Back Bar Tunisian Simple StitchBack Bar Tunisian Simple Stitch
Twisted Tunisian Simple Stitch
Another variation on the simple stitch is made by inserting the hook from left to right. When you insert the hook this way there are two was to twist the stitch - up and down - and the two stitches look very different.
Twisted Down Tunisian Simple Stitch Notice that the vertical bar is pulled to the left of the hook and there is a twist at the top of the stitch
Twisted Up Tunisian Simple Stitch Notice that the vertical bar is pulled to the right of the hook and the stitch appears to slant to the right
Comparison of Tss & Twisted Down (left box) and Tss & Twisted Up (right box) stitch combinations