#include<bits/stdc++.h>

using namespace std;

int t, n, k;

string ans[1005];
void verify(int prek) {
    int res = 0;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++) {
            if(i != 0 && ans[i][j] != ans[i - 1][j]) res++;
            if(j != 0 && ans[i][j] != ans[i][j - 1]) res++;
        }
    assert(res == prek);
}
void solve()
{
scanf("%d %d", &n, &k);
if ( n == 1 )
{
puts("Possible");
cout << "B" << endl;
return ;
}
if (k == 1 || k == 2 * n * (n - 1) - 1)
{
puts("Impossible");
return ;
}

if (n == 2)
{
puts("Possible");
if (k == 0)
{
cout << "BB" << endl;
cout << "BB" << endl;
}
else if (k == 2)
{
cout << "RB" << endl;
cout << "BB" << endl;
}
else if (k == 4)
{
cout << "RB" << endl;
cout << "BR" << endl;
}
return ;
}

for (int i = 0; i < n; i++)
{
ans[i] = string(n, 'B');
}


if (k == 0)
{
;
}
else if (k == 2)
{
ans[0][0] = 'R';
}
else if (k == 3)
{
ans[0][0] = ans[0][1] = 'R';
}
else if (k == 4)
{
ans[0][0] = ans[n - 1][n - 1] = 'R';
}
else if (k == 5)
{
ans[0][1] = ans[n - 1][n - 1] = 'R';
}
else if (k == 6)
{
ans[0][1] = ans[n - 1][1] = 'R';
}
else if (k == 7)
{
ans[0][1] = ans[n - 1][0] = ans[n - 1][n - 1] = 'R';
}
else
{
for (int i = 0; i < n; i++)
{
for (int j = (i & 1) ^ 1; j < n; j += 2)
{
ans[i][j] = 'R';
}
}

int tot = 2 * n * (n - 1);
if (tot == k)
{
puts("Possible");
for (int i = 0; i < n; i++)
{
cout << ans[i] << endl;
}
return ;
}
else if (tot - 2 == k)
{
puts("Possible");
ans[0][0] = 'R';
for (int i = 0; i < n; i++)
{
cout << ans[i] << endl;
}
return ;
}
if (k & 1)ans[0][1] = 'B';
tot -= 3;

vector<pair<int, int> >_change[5];

for (int i = 0; i < n; i++)
{
for (int j = (i & 1); j < n; j += 2)
{
if ((k & 1) && abs(i + j - 1) == 1)continue;
int sub = 4;
if (i == 0)sub--;
if (j == 0)sub--;
if (i == n - 1)sub--;
if (j == n - 1)sub--;
_change[sub].push_back({i, j});
}
}

int s2 = _change[2].size(), s3 = _change[3].size();
int s4 = _change[4].size();
while (tot - k >= 6 && s3 >= 2)
{
tot -= 6;
int u = _change[3].back().first, v = _change[3].back().second;
_change[3].pop_back();
ans[u][v] = 'R';
u = _change[3].back().first, v = _change[3].back().second;
_change[3].pop_back();
ans[u][v] = 'R';
s3 = s3 - 2;
}
while (tot - k >= 2 && s2)
{
tot -= 2;
int u = _change[2].back().first, v = _change[2].back().second;
_change[2].pop_back();
ans[u][v] = 'R';
s2 = s2 - 1;
}
while ( tot - k >= 4 && s4)
{
tot -= 4;
int u = _change[4].back().first, v = _change[4].back().second;
_change[4].pop_back();
ans[u][v] = 'R';
s4 = s4 - 1;
}

if (tot != k)
{
puts("Impossible");
return ;
}

}

puts("Possible");
for (int i = 0; i < n; i++)
{
cout << ans[i] << endl;
}
verify(k);
}

int main()
{
scanf("%d", &t);
while (t--)
{
solve();
}


return 0;
}