Understanding grid negative values

Question

I'm trying to make a grid that has a full span row at the bottom.

For full span columns I can use grid-column: 1/-1.

For single span columns I can use grid-column: 1/1.

For single span rows I can use grid-row: 1/1.

But if I want to define the last column or row, I have to write grid-column: -2/-1.

Why is the syntax not the same as with 1/1 for the first column/row? Or am I making a mistake somewhere?

I also made a jsfiddle to demonstrate my problem: jsfiddle

.grid-container {
  display: grid;
  grid-template-columns: 5px 1fr 1fr;
  grid-template-rows: minmax(50px, 2fr) 1fr 1fr 1fr 1fr 1fr 15px;
}

.grid-item {
  width: 1fr;
}

.header {
  display: flex;
  grid-column: 2/-1;
  grid-row: 1/1;
  justify-content: center;
}

.border-left {
  background: purple;
  grid-column: 1/1;
  grid-row: 1/-1;
}

.border-bottom {
  background: #410266;
  grid-column: 2/-1;
  /* grid-row: -2 / -1;  this will work, -1/-1 will not */
  grid-row: -1 / -1;
}

<div class="grid-container">
  <div class="header"> HEADER </div>
  <div class="border-left"></div>
  <div class="grid-item">1 </div>
  <div class="grid-item">2 </div>
  <div class="grid-item">3 </div>
  <div class="grid-item">4 </div>
  <div class="border-bottom"></div>
</div>

Answer 1

when using the same value inside grid-column/grid-row you will fall into this rule:

If the placement for a grid item contains two lines, and the start line is further end-ward than the end line, swap the two lines. If the start line is equal to the end line, remove the end line.^ref

So saying grid-column:-1/-1 means grid-column:-1 which is grid-column:-1/auto

auto

The property contributes nothing to the grid item’s placement, indicating auto-placement or a default span of one. (See § 8 Placing Grid Items, above.)

So basiclly you said to your element to start at the last line and span one column which will create an implicit new column:

A basic example to illustrate:

.box {
  display:grid;
  grid-template-columns:20px 20px 20px;
  grid-auto-columns:100px;
  grid-gap:5px;
}

span {
  grid-column:-1/-1;
  height:40px;
  background:red;
}

<div class="box">
  <span></span>
</div>

You can see that the span is having 100px which means it create a new column inside the implicit grid and is not inside the explicit one defined by 20px

When using -2/-1 it's clear that you will consider the before the last and the last line and the element will be placed in the last explicit column:

.box {
  display:grid;
  grid-template-columns:20px 20px 20px;
  grid-auto-columns:100px;
  grid-gap:5px;
}

span {
  grid-column:-2/-1;
  height:40px;
  background:red;
}

<div class="box">
  <span></span>
</div>

Same logic apply when using positive value but you won't notice a strange behavior since you will most likely span an explicit column thinking it's correct to specify, for example, grid-column:1/1